Very interesting.. It reminds me of Howl's Moving Castle.

I would change up the theme so that these are floating castles and there are propellers on the bottom and sides to indicate that it's flying. I might even do my own version of this.

You went with a very ambitious design for your first map. I think for the next project you should concentrate on a smaller map and really refine the details till you're happy with them. For example, I saw a bunch of improperly-angled blocks in the map (such as in the doorways and bookcases in the corners). I don't see any value in the bottom level with the healthboost - it seems like a waste of time so I'd suggest scrapping it. Also the ground has not been detailed in any way so it remains much the square that you start off with in a default map.

The staircases are not designed with playability in mind. It's very easy to smack your head on the bottom of them since you made them gridlike rather than ramps. I'd suggest smoothing them out or using noclip. Also the top of the stairs has an additional wood platform on them for some reason, so you compensated by angling one of the steps.. It looks ugly. If you need to compromise with the map like that, you're doing it wrong. An ideal design will fold out organically without any need to compromise.

There was opportunity for sniping spots, but the walls are so tall that it's not practical to just walk up to the windows and shoot; you need to jump into the windowsill which is committing for a camper.

The lighting needs work. One castle is almost entirely shrouded in dark while the other one receives most of the sunlight. If this ends up being castles in the sky then the sunlight will need to be more powerful.

Texturing is weird. The bookcases, for example, don't look like they're in the right spot, and you have very repetitive (wood, etc) textures that make this overall boring.

I credit you this: the inside and outside of the castles is pretty epic to look at, and the theme is quite creative. I don't see the theme properly established yet; it doesn't tell a 'story' but you can definitely fix that.. Much of my criticism is in the actual layout of the castles though which is really difficult to fix.

Anyways, 3/10. Not a strong release technically, but artistically this has some interesting applications.

the bottom healthboost layer was designed so you could jump there and be safe from snipers. Otherwise, I'll see what I can do.

Do you have any advice on gameplay?

Texture choice like this is not common it surprised Me to see it used!

interesting map, very original theme ^^!
the texturing of the map is sometimes a bit wacky, because in some places they arent fitting at all. i found out you copied and paste a lot of stairs, be sure you change the direction of the texture when doing such thing (so just retexture those pasted stairs, if the wood is pointing to a very strange angle).
i would recommend that you change the odd looking wooden planks all over the map, for some other that look more realistic (setshader glowworld ... is an option to change the colour of an other wooden texture to some more "reddish" texture, that you still get the same effect, but then with a more realistic texture).
the geometry isnt very good. you could have done the stairs lot better (1 plank of the stairs is 1 small cube lower then it should be. this would fix the problem with the upper side of the stairs, where you just made the last plank diagonal so its fitting the next floor. this isnt a good solution because it lacks flow). the circle at the quad could be done better, because its more like a pentagon shape (of course im not expeting you make a perfect circle, as long as its less blocky and round then its now).
there are some platforms inside rooms that arent getting supported at all (like a full wooden platform), wich is very unlogical.

I'm not so sure about the chains, tho... they didn't really make it look any more "logical", but I'm not so sure if I want to completely get rid of 'em.