Hi neatt. Your map is pretty good. The lake, which is the center piece of your map looks very nice. The cat tails fit right in. The brushwork is good. Also nice use of displacements for the ground and lake. I really like the jagged edges of the walls. Nice touch.
One thing I did notice is that when a player climbs up or down the steps, they hit their head on the ceiling. Not enough to get stuck but it is noticeable. I think you should give some thought to the outer walls. Maybe use the displacement tool to create some rock formations to give the appearance of realistic outer walls. You picked a good texture that would look great if turned the flat walls into some displacements.
You could think about adding some more props or broken walls around for players to have some additional cover. The surrounding areas are a bit open.
This is a very good effort and defiantly see a lot of promise. Nice job.