The moss growing more and more as the years go on is a great detail. I didn't manage to get further than 25 years, although I gave it plenty of good tries. The grey enemies blend to the dark background making them a bit hard to see. I had some success trying to get the enemy hordes stuck behind corners, but it wasn't enough to get me through the level.
There seemed to be a slight performance problem, as the game froze for a couple of seconds whenever loading a new effect for the first time (first time I shot, first time enemy died, first time I got hit), but luckily it didn't happen more than once each.
Congrats on making a fun game in such a short time!