Terra God is a rather crummy (and nonsensically named) tower defense game that brings exactly one interesting feature to the table.
Most of the game is bog standard, although it does have a rather restrictive "can only build on 'controlled territory'" feature similar to that in Demons vs Fairyland, wherein you can't build towers if you don't have other towers nearby already. The way this particular game works, you actually automatically must start with a tower or you can't ever build any; makes one wonder how that first tower got put in place.
Anyway, the game doesn't feel especially well-balanced to me, with there being a drastic difficulty spike around the fourth or fifth stage, but it does have one feature that's interesting enough to make note of: There are "enemy towers" on the map that actually help the enemies.
This game's honestly kind of terrible and not really worth playing (it's actually actively annoying, what with the tower quotes that spam when you click on them), but the idea of there being "enemy safe zones" (I've seen healing towers and armor-boosting towers) on the map is kind of conceptually interesting, and I'm curious what a decent game that used the idea might be like. (It does have the potential problem of making one's own tower placement even more critical than it was already, but at least the game doesn't limit tower placement the same way the Incursion/Kingdom Rush family does... even if it has its own way that's not really very good either.)
In a pathing/mazing tower defense game, it'd be kind of interesting to see enemies who "dropped" towers when they died, for that matter. There's probably quite a bit of conceptual space to explore there.