Basically includes a block-like obstacle, corners for the edge/interior of the arena, as well as a sprite for the tree you'll be defending. I made a quick mockup to show how they might look when applied to the field.
With proto tiles laid over.
Ran into several hurdles trying to get all of the edges to fit together perfectly... which is, I guess, the most crucial element of a tileset, so it makes sense that it'd take the most effort. The other main thing is making sure they all had a unified look with the trees and Towers so the outlines needed to be about the same weight. As for the new tree sprite, a little lighting suggests the glow radiating from the approaching horde of enemies, and a more top down perspective matches the angle of the other elements on the field.
We can hopefully use this template to create pretty much any environment we can think of, with different types of trees and wall material depending on the setting. Old Western, frozen tundra, outer space... who knows?