This was a really interesting interpretation on the theme, I don't think I've seen anything similar to this yet. Your graphics are simple and it was easy to see what was happening on screen, and the controls were pretty straightforward and I understood how the game worked pretty quickly. The sound effects are funny as well, although maybe next time make sure that the sounds play through both headphones, I got quite some chills from that haha.
Only thing with controls though is that I never really found a need to use the spacebar to drop a gravestone, I more or less just found a random stone, picked it up, and dropped it off. It would be nice to have a need to use the spacebar (For example, puzzles where the gravestone has to be dropped before entering and unlocking a passage). Also, for some reason when a zombie is entering the grave and not chasing the player, the player can still collide with it and trigger a game over, which is a little bit troublesome (And resetting the entire game when getting hit was a bit frustrating too).
Overall, I still really like this entry, and I would want to see a more polished, expanded version of this in the future as well!