Here's a level I made earlier today, "The Mansion". This was actually a concept I came up with very early during this pack's creation, but I couldn't get things to work properly at the time.
Some very careful considerations were needed to make the visual concept behind this level actually work, without the issue of "things look weird from certain angles" or "haha, face render limit strikes again". Though the somewhat boring appearance is more a limitation of the texture set than any technical limitation - the Maze 1 texture set really doesn't offer all that much to work decoratively when creating structures more than one or two blocks high, and Maze 2 and Maze 3 don't really offer much full stop. (This concept almost might've worked better for a Castle level, now that I think about it...)
This is a Fun level, so it's almost an X-of-everything-with-lenient-save-requirement... except you don't get given any turners.
The "mansion" is actually only two blocks deep - the front layer, normal terrain, and then one more layer, which is indestructible. It's block 3'd on the back.
Of course, that alone would result in the user being able to just move the camera around to the back, and see that the rest of the mansion isn't there. This is avoided by fencing the camera in with a wall of block 4's - and on this level, block 4 is marked as non-solid to lemmings.
But then what about the trees? Those actually use block 3 instead, which can be left as solid-to-lemmings. Since the trees aren't touching any block faces, the side effects of block 3 don't cause any issues here.
Oh, but the trees in the same place as the camera border - they need to be solid to both lemmings and the camera! Those in fact just use a regular block with all invisible faces - there's only two such trees, so this doesn't really pose much threat in terms of face render limit.
Of course, for all this to work, the level also has to have a fixed preview camera (despite not being enclosed), and lemmings also must not be able to quite reach the edges / above the top of the mansion (via the border kill points) - otherwise virtual lemming could break the immersion. And yes, I tested things like "what if you build as high as you can, while also trying to get quite far from the mansion, then assign a last-second blocker before getting zapped and turn the camera towards the mansion during those last few frames?" - the level's boundaries are set to avoid being able to see anywhere the player shouldn't even when doing things like that.
Ultimately, this is really set up as an indoor level, with the mansion as one wall of it - except that the enclosed area is huge, and three of the four walls, as well as the top, only restrict camera movement, not lemming movement (though lemmings get zapped if they go much further).