Comments

You have many invalid solids - all the tower roofs need to be redone. Redo the outer walls to just meet the sky at the tops - don't put a skybox completely around your map like you did. Carving and "make hollow" are more no-no's which you did as well. The towers are kinda too complex: I'd simplify them and raise them 1 unit off the 'floor' to prevent the feet from chopping the floor into hundreds of sections.