While the map looks well made, I still don't see anything that separates it from any other TD.
-Add another builder unit that can build towers in the paths. These towers would be small and weak, but would be useful in combination with the towers that are built on the higher parts. I haven't seen a TD yet that has done this.
-Give the builder units an inventory and allow them to buy some items. There could be a shop that sells items like mines (maybe even elemental mines).
Most TDs are boring because of the lack of interaction. You just build a few towers and hope they kill all the enemies. If you use these suggestions, I think they would allow players to do more.