Had a quick run-around on your map. You pulled off the graveyard-theme very nicely and you should keep the dark-green setting. You've put great effort into building that mausoleum. But i also think that the map is too big, flat and open. For example, the mausoleum itself is really empty and it could benefit from shrinking it down. In return you can add more to the interior and flesh out additional pathways such as underground passages or broken stairways to the top. The same applies to the outside area. Using the graves as cover is a nice idea, but you could have done so much more with it. Create some interesting terrain with slopes and hills....maybe open graves as spawn points for the zombies ;). The possibilities are endless, just look at some pictures of underground mausoleums.

I hope this actually helps you ;)