IRF

Isn't 'level' more of a reference to the degree of difficulty (in e.g. Manic Mixup, once you've completed all the caverns the first time through, the difficulty moves up a level and you go through them all the second time)?

Yes, I've experienced it too. It is an additional difficulty, and it is frustrating 😵.

That's a nice-sounding definition, although I think it might lead to using the words "cavern" and "level" interchangeably and always referring to MM games, as there are no such levels in JSW.
However, it won't always work, I believe, for the following reason: In JSW64 and other advanced game engines you can have a MM-like room which Willy enters and can only leave via a portal after collecting all items in that room. However, such a room may be not a part of any longer sequence (like having to go through all of the original MM rooms before returning to the main, JSW-like part of the game map), it may be there on its own, as a kind of "detour" among JSW-like rooms. In this case, it would still be a cavern as per Ian's definition, but I don't think anyone would call it a 'level' (it would be a 'room' and a 'cavern', but not a 'level'! 😜).

I suppose I should have said (and I think it's what I meant) that a cavern's only means of exit is via a portal.
In the JSW context, "level" could be considered as a reference to which floor of the mansion each room is on. (Battlements on the top level; Wine Cellar on the bottom level.)
Edit: With apologies to the Watchtower and the Entrance to Hades!

I should just add that, notwithstanding the name 'Eugene Lair', this cavern is more like a hybrid, conceptually speaking, of two original Manic Miner caverns:
- 'The Final Barrier' (aesthetically speaking, with the action all taking place in the bottom half of the playing area - although a strategically-placed pair of dancing pixels give the appearance of moving eyeballs to the [otherwise static] giant 'Eugene' printed at the top of the screen); and
- 'Skylab Landing Bay' - the fireballs are a very inventive and fiendishly tricky variation on Matthew's original Skylabs (they appear to be ascending rather than descending, and for most of their traverse they are INKless).
The version of the cavern which ended up in the official release of 'Ultimate Manic Miner' (with the 'Skylab' fireballs staying in their columns and not getting horizontally displaced, due to a flaw in the JSW64 game engine) really didn't do Igor Makovsky's original design justice.  Hence I was so determined to fix it!
For all of the above reasons, I really recommend people try out 'Eugene Lair as Manufacturer Intended' (via the above method, taking one of Danny's recordings as a starting point and 'seizing control' of the gameplay), even if they don't fancy traversing through the rest of 'Ultimate Manic Miner' to reach it.

Waiting around too long can have two potential consequences: you might get hit by a fireball flare-up, but even if you manage to avoid that, if you crumble away all the cyan platforms near the portal, you may end up stranded on the lower level, with the flashing portal beyond reach! In fact, I thought you had made that mistake in one of your recordings (but it turned out that there was one crumbly cell with a pixel-row or two remaining, just within reach of the portal, so you managed to sneak out of there!)
**
Another unexpected difficulty can arise from the 'shadow' of the Skylabs passing over a column shortly before the deadly fireball arrives in that column. It can provide a helpful warning to take swift evasive action, as it briefly turns Willy's sprite yellow in advance of the deadly yellow pixels bursting out. But it can also make items temporarily uncollectable, if you try to jump up to collect them just at the moment that the yellow shadow passes through them and Willy! (Willy needs to have white INK in order to collect items.) It happened to me once, so I had to go back and try again for the item in question. Deliciously frustrating!

Well done, Danny! I agree, the sense of panic is only increased if you don't have Rollback to rely on!  I think I did manage to do it once though (eventually!)
Because these are RZX recordings, I would suggest anyone who wants to have a go at this fiendish cavern 'as author intended', all they need to do is:
- load up one of the recordings in SPIN;
- once the gameplay starts, select 'Stop Playback' from the 'Recording' drop-down menu at the top of the emulator window;
- walk Willy leftwards into the portal of the start-up room (if that hasn't already happened by the time you stop the playback) ;
- emerge from the portal into 'Eugene Lair', and BEWARE THE FIREBALLS!!! 😜

Ian has provided a fixed version of "Ultimate Manic Miner", where both the Eugene Lair and some other issues are fixed. Attached are three playthroughs of the fixed "Eugene Lair" done using the modified file. I recorded them using the Rollback feature.
To be honest, when one plays using Rollback, I would say it's not a particularly difficult challenge - when you die, you roll back and try doing things differently, either by improving your movements (e.g. when you see you just need to move a little bit more quickly to get things right) or by choosing a different way ('let's go right instead of left at this time') or timing ('let's not go for those items now, but wait until the horizontal guardian has passed'). Of course the Rollback makes all the difference - playing this room without its assistance would certainly be a nightmare.
UMM fixed Eugene Lair 3.rzx UMM fixed Eugene Lair 2.rzx UMM fixed Eugene Lair 1.rzx

This distinction would be crystal-clear between the original "MM" and the original "JSW", but later games - like the ones based on the JSW64 game engine or Norman Sword's "Manic Jet Set Willy" blur it IMO. In the original "MM" the (activated) portal was the only way to leave the room, and in the original "JSW" there were no portals. However, in the later games you can have a room with a portal that takes you elsewhere, but you can also leave this room by "normal" means, going into one or more of the adjacent rooms. In this scenario the portals merely work as teleporters, to get you elsewhere instantaneously. I think it's questionable whether a room should be called a cavern only because it has such a portal.

Good point about who entered the room. Though I guess you could have said "... after he entered the room [or cavern]". The player isn't really entering the room themself, just guiding Willy into and through there! 
To me, the distinction between rooms and caverns is that the latter have a portal for egress.

I guess many people like to stick to the 48K limit. (If you're going 'retro' with your games, then you may as well go the whole hog, and the 128K Spectrum came later than old rubber keys - although the ZX81 was even earlier, its 16KB limit perhaps takes that concept too far/limits the size of the games too much?) If you add up the MM(48) and JSW48 mods from your list above, they amount to about two-thirds of the total (with JSW128 and JSW64 games collectively only making up about a third).
So with that in mind - now that Norman Sword has managed to devise an optimised MM game engine which allows significantly more caverns within the 48KB constraint (how many more exactly? I don't believe Norman specified?), perhaps more such modified, more expansively cavernous games might be forthcoming in future?

I wonder if all the other 'tributees' are aware of their 8-bit representation in the Manic Miner Pub in UMM?

Do you want me to DM you a copy of the fixed gamefile, with the starting room/position set to the appropriate place?

Sorry, I should have said 'The Mine Vehicle Department' there, not the MM Pub. 

Good point about who entered the room. Though I guess you could have said "... after he entered the room [or cavern]". The player isn't really entering the room themself, just guiding Willy into and through there! 
To me, the distinction between rooms and caverns is that the latter have a portal for egress.

Thanks, Ian, this is the kind of feedback for JSW Central I greatly appreciate! (and receive rarely) 👍.
I have corrected the entry on JSW Central for "JSW64: Manic Miner" according to your suggestion (adding "in some circumstances"). On second thoughts, I also modified the end of this sentence to read: "the first time it comes Willy's way after the player entered the room" (instead of "after entering the room"), because I believe that "after entering the room" would actually refer to the guardian rather than to Willy ("the first time it comes Willy's way after entering the room" = "the first time it comes Willy's way after it entered the room", "it" referring to the guardian). I'm not 100% sure my feeling is correct here but I hope the modified version is OK.
As for "caverns" having to be used when referring to MM games and "rooms" when referring to JSW games, I don't perceive this as a definite rule. "Rooms" are generic for me, suitable for both games. "Caverns" are more MM-specific, I wouldn't use them for a JSW game unless the room in question actually depicted (or was called) a cavern. That's just my personal feeling about it 🙂 .

 

Yes, I'll definitely do that. I will upload it right here.
 
That's correct. My 'policy' for making the walkthroughs is to complete each game without losing any lives (unless necessary to collect an item or more) and doing it as efficiently as possible, resulting in a score in a MM game that will be as high as possible or the completion time in a JSW game that will be as good (early) as possible.*
As a result, if there are rooms in JSW games that the player doesn't have to visit in order to complete the game, they may be omitted in my walkthrough. "May" (not "will") because it depends on whether going through this room/these rooms is propitious to completing the game as efficiently as possible. If it is, you will see them in the walkthrough. If it's not, they won't be showcased.
The two rooms you've mentioned fall into the latter category and therefore are not present in my walkthrough. Nevertheless, I am grateful to Igor for this personal tribute (I am positive I'm "the Danny" it refers to 😉). The absence of the room in the walkthrough is a testament to the fact that I do stick to my 'policy' while creating the walkthroughs 😉 .
* "as efficiently as possible" within reasonable limits. I do not strive to achieve the maximum possible scores, like the ones determined (scientifically and via a 'peer review' LOL) for "MM". They have not been determined for the other games, so I don't know what they are. And even if I knew, producing pixel-perfect walkthroughs would require an investment of significantly much more time and energy than producing just 'decent' walkthroughs. I would not consider this 'investment' justified.

At the very least, will you consider doing a play test of the UMM room 'Eugene Lair' with my Pokes applied (so it's 'as manufacturer intended')? Maybe you could upload it here at JSWMM if you don't think it belongs on your YouTube channel?
***
Incidentally, your walkthrough doesn't display a couple of 'Easter Egg' rooms in UMM which it isn't necessary to visit in order to complete the game (namely 'Manic Miner Pub' and 'The Mine Vehicle Department'). The latter of which contains a personal tribute (in the form of labelled sprites) to several of the 'old skool' people from the MM & JSW scene, including yourself! (Unless there is another 'Danny' on the scene?)

Another consequence of the bug in 'Ultimate Manic Miner'...
There is another room in the game (other than 'Eugene Lair') which contains Skylabs, namely 'Reservoir Pumping Station'.
However, the game is quite linear in its layout, and following the usual route through the game, the player would encounter 'Reservoir Pumping Station' before any of the rooms which contain Droplets (namely, as far as I am aware, 'The Menagerie', 'The Stonehenge Mine Subway', and 'The Bank').  Therefore the corruption of the Skylab code will not have taken place by the time you reach 'Reservoir Pumping Station' after you first load up the game file.
However, if you were to play the game up until one of the rooms containing Droplets, but then lose all your lives and start another game, then when you reach 'Reservoir Pumping Station' the second time around, the gameplay will be different than it was the first time through!  (Because the Skylabs in that room will have morphed into Droplets i.e. always dropping down at the same x-coordinate, with no variation to their horizontal 'dropzone' as is the case for classic Skylabs.)
This is reminiscent of the bug in certain Manic Miner mods where Willy jumps off the top of the screen, causing overwriting/corruption of some of the data for other caverns.

Daniel, when you do get round to completing 'Ultimate Manic Miner', I recommend you use the version which I patched to fix the bug that affected the room 'Eugene Lair', which caused the Skylabs in that room to behave as Droplets (ie they no longer varied their horizontal position). My fix (which required a change to the game engine) brought the game in line with the author Igor Makovsky's original intentions (as expressed in a comment by him on the Yahoo! MMJSW forum).
Come to think of it, I don't think that patched version ever had an official release? I was in email contact with Igor about it at one point, he expressed gratitude for the suggested fix, and from memory had an intention to update the game himself at some point, including the fix. But that was several years ago now and I haven't heard anything from him since. My fixed version is probably attached to a topic here somewhere, but it would be nice if you could include it in the relevant entry at JSW Central?
UPDATE: I just checked, and I didn't actually attach a bug fixed gamefile in the topic that I created on this subject* back in the day. Rather, I included a list of the POKES which may be used to fix the game engine to allow for both functional Skylabs and Droplets in a single game. (John Elliott's Droplets patch for JSW64 permanently erases the code which determines the Skylabs' behaviour).
 
*The topic is called 'For fans of Ultimate Manic Miner', which in hindsight seems somewhat ironic (given Danny's subsequent description of how much criticism the game got when it was originally released - for its overuse of quirky features - which I wasn't aware of when I created the topic!) 

It seems that the above suggestion came too late for your initial walk through of UMM, as I've just noticed that you did complete it and uploaded a video of it to your YouTube channel.
However, the 'Eugene Lair' in that version looks a lot easier than (and not in the spirit of what) Igor originally intended*, so perhaps another walk-through if the bugfixed version could be put on your to-do list?
*Those volcanic flares are supposed to shoot up at different points in the cavern, in the same way that Skylabs drop down at different horizontal positions, so that nowhere is safe to stand for long!
For reference, the room in question can be seen 26 minute in to this video:
Now watch those flare-ups - because of the big they only pop up in two places - imagine trying to navigate around whilst the fireballs continually switch x-coordinates - a whole different proposition!! 

Yes, that's correct. "JSW64 Manic Miner: James Bond" is not the best example to consider, because its number of rooms is indeed specifically conditioned by the output of the movie industry.
Having said that, the reality - and my main point - is that people have not been creating MM-style games with significantly more than 20 rooms/caverns even though, for many years now, they have had the possibility of doing just this, using one of the JSW64 game engines and JSWED. And this, I believe, suggests that it is not the 20-rooms-only limit that has prevented people from creating more MM-style games, but rather some other factors.

That was exactly my intention, in a positive, non-critical spirit 🙂 .

My reading of Danny's post wasn't that it was a criticism of your endeavours, more of a lament that it isn't (currently) presented in a more accessible format that would allow people to create games using your nicely optimised MM engine with ease.
I do seem to recall you releasing a version of Manic Miner with a fairly user-friendly in-built editor? But that was incorporated within the 48KB of the game itself, if I recall correctly, which I guess limits the amount of additional caverns that could be included within the 48KB memory limit? Danny is suggesting (perhaps as a pipe dream, but I believe in a positive, non-critical spirit) that someone [else, if not yourself] might at some point create an editor which runs on a PC or other similar platform, incorporating a user-friendly GUI, and into which 48KB MM game files (optimised as per your recent upload) could be loaded and edited by Joe Bloggs.

I would just add that, in relation to Andrew Broad's 'JSW64 Manic Miner: James Bond' game, the determining factor for the number of caverns was the number of (canonical) James Bond movies that had been released at the time of the game's publication.  (Each cavern being dedicated to a single Bond movie, in chronological order.)
The original version (1.0, released in 2007) contained 21 caverns, the last one being 'Casino Royale' (which was the most recent movie at that point, having been released in 2006); the game was updated with version 1.1 in 2009 to include an additional cavern 'Quantum of Solace' (film released in 2008).
In the readme file released with v1.1, Andrew Broad said: "I plan to extend my game as future Bond-films are released. At the time of writing, Bond 23 is due in cinemas in 2011, so that will be added in v1.2" [However, in reality the next movie wasn't released until 2012, followed by two more in 2015/2021, and no further versions of Andrew's game have been released to date to incorporate caverns covering those movies.]
So in the hypothetical world where more than 30 James Bond films had already been released prior to 2007, Andrew would probably have taken the opportunity provided by the JSW64 game engine to create a MM game containing more than 30 caverns from the outset.