Overall art is pretty good, add some death sounds for zombies. mana regens way too slowly at the beginning, once you upgrade it, it has 0 cd so you just spam attack. AI gets stuck behind solids (tombstone, rocks). Also it took me 5 mins to search for all the zombies on the map. Health bar doesn't reset properly on new wave's.
Your game just needs some fine polish, overall pace is too slow.