This is awesome.
I once wanted to make something like this, but never thought of using actual blockheads, I wanted to do a simplified form, something like a Roblox figurine with a block head (don't bash me, it's just the basic idea of it for making things less detailed due to them being small and all)
Mostly because I think they will lag if used in a large amount... but really this is beautiful either way!
You could use more styles of trees, though. Also, how about adding the tank, cannon and rowboat?
You could've done the following edits on the minifigs to make them less resource-cramping: Square-ify their shoulders, remove the spaces between fingers on the hands, simplify the shoes with just a hollowed cube and pelvis just as another cube that goes inwards on the bottom.
Normal:


Edited:


Perhaps better shoes, but for them being miniature there is pretty much no notable difference that makes someone spaz out, despite perhaps halving the verticles and quads.