Displacements do not block the placers line of sight so anything behind them is renders as well. This is mainly caused by your displacements. Here is an example screenshot (I have wireframe turned on) This is bad because it can cause a huge drop in framerate. At any one given point in the map, nearly every part of the map is rendering at once.
In your map, there is almost no optimization. Optimization is key to making your map perform the best it can and to give players the best experience possible. The first and most important thing that I want to hit on is the lack of optimization.I also like how you added some more details to the walls like adding those archway things (I have no idea what they are called) they really help complete the overall theme of the map.But, make sure you sew them because I noticed a few gaps when I was walking through the map. They look a lot cleaner than before and they aren't overlapping each other is weird places.