'Mid-size' dungeons don't make much sense. Anything that tries to cover levels 1-3, for example, which isn't in itself vast and flat in relation to depth, is probably a pointless death-trap if it's not a pointless distraction.
Consider the First Level Party. What they need is an environment - or series of environments - that will allow them to 'level up' before going down to (dungeon) Level 2. They can do that by visiting a bunch of small Level 1 cave systems or temples to evil, and killing or driving off the monsters and taking their treasure; or they can do that in one great big Level 1 dungeon. Whichever, they need to do loads of it. For a 5-PC party, there needs to be at least 10,000XP (more like 12,000 or more) that the party can reasonably be expected to get hold of before they can feel confident about getting busy on Level 2. OK, short raids maybe but it's pretty dangerous for First Level characters on Level 2. Level 1 should be pretty dangerous really, and Level 2 should be very dangerous. First Level characters should therefore be able to spend a lot of time on Level 1 (either a single vast Level 1, or a whole bunch of Level 1s in different locations, I don't care, in my opinion there's nothing wrong with a succession of lairs). Really it's not even that they 'should be able to' - they need to spend a lot of time on Level 1.
Dungeons with a few Level 1 rooms and some Level 2 rooms and some Level 3 rooms are therefore pretty pointless 'dungeons'. The First Level party cannot gain enough experience on Level 1 to make the upgrade to Second Level. So there is no point, at that location, in having a Level 2 or 3, unless you've already taken the party through a series of lairs elsewhere and they're transitioning between First and Second, or Second-Third. On the other hand, why does a Third Level party need to go through Level 1, or even much of Level 2?
"So what?" I hear you cry. "Who runs dungeons like that anyway?"
Well, I've been trying to build the 'Haunted Keep' from the Moldvay Basic Rulebook - to stand in for the Haunted Keep near Luln in the Duchy of Karameikos from the Mentzer Expert Set, which I have no information on, not being in possession of the Gazeteers. Here's the pic - a lovely pic I have to say, I think it looks great, which is what led me to try to reproduce it for a party to adventure in:
Original image by Erol Otis, (c) TSR 1981, now owned by WotC.
I found the image over at http://3d6trapsandthieves.blogspot.co.uk/2012/07/ism-inspirational-source-material.html - so thanks for that, I couldn't to bothered to go upstairs and find my Basic Rulebook and then scan the illustration.
As I say, in my opinion a great-looking small, multi-level dungeon.
But completely unplayable for a First Level party.
Even with the descriptions of play in the book (there are several rooms described and more hinted at on 'Level 1', which is the ruins), and the notes as to how to run an adventure, if that's a Levels 1-3 dungeon (as text makes plain that it is), there is no possibility that a First Level party could gain enough experience in that setting to make the trips down to Level 2 and Level 3 feasible. At best, if you count the ruins, and all of the 'dungeon' parts (as opposed to the 'cavern' parts) as Level 1, then maybe - maybe - you could get a small party up to Second Level to attempt the clear-out of the caverns. But I'm not convinced.
Now, to be fair, the descriptions of play in the book say that the example party is in a transition between Second and Third Levels. Thus, Level 1 should be a bit of a cake-walk, actually.
So, consider the Second-Third Level party. What they need is a deep hole. Something that will take them all the way beyond Level 1, to a part of Level 2 with access (and exits) from Level 3. But it looks to me like the pictured dungeon has access from Level 1 (ruins, where the party has no challenges at all) to something where they don't really know if they're on a Level 1a (the first underground sublevel of Level 1, in which again there are no significant challenges) or actually on Level 2; either way this can then be used to bypass most of Level 2 (where the party really wants to be) to get to Level 3 (that the party wants to be able to easily escape from).
One thing that 'The Haunted Keep' needs to make it work as
a Level 2-3 mini-dungeon is an exit, to a point outside the ruins, that
will allow Second-Third characters to gain access to a corner of Level 2
which they can use as a base to explore Level 3. What it doesn't need for such a party is any real detailing of, or time spent in, Level 1. On the other hand, if it's being run for a First Level party, Level 3 is too far to venture and Level 2 should only be considered if the party is feeling very confident indeed.
I will run the Haunted Keep, and I'm going to keep everything I can from the section-drawing and the accompanying text. But I'm not going to run it in the form I've currently written it up. At the moment it's a death-trap for First Level characters venturing too deep (alternatively, it's something they'll raid the surface of and completely ignore the lower levels, and this is how I'm now hoping to make it work) and it's a pointless waste of time for Second and Third Level characters who are forced to trudge through endless encounters with impoverished Goblins to reach the points where they can start to access the good stuff.
So, off to the files for a re-write; perhaps this time I'll get it right and can use the ruins and perhaps upper underground level for my beginner party to accomplish a lightning raid in a 'rescue the prisoners' mission (currently those prisoners are languishing in Level 3 - the text says they will be 'scattered throughout all of the levels; however, the most important prisoners must be rescued from the centre of the wererat lair' which can only work for a Second-Third party); and later, when they're really at Second-Third Level, they can perhaps have a return visit and see what lies down those staircases that I hope they won't want to explore the first time - by which time I might have decided what exactly is going to be down there to interest them...
Edited for the sake of clarity as to which Levels I'm actually referring to...