Great fun although here's a suggestion or two .
1. In the beginning when you instruct the player to pick a spawner to begin, maybe highlight the spawners themselves to indicate what the spawners are. It's not the end of the world and to be honest this is a bit of a non issue but I spent an extra few seconds clicking between the large nexus and the spawners themselves before realizing the smaller ones were the spawners.
2. There's a bit of a difficulty spike after you destroy the 3rd/4th enemy nexus, all of a sudden you're flooded with enemy bots along with the new bot type. Maybe have the AI gradually increase its spawn rate as you destroy more of their towers instead of all at once?