also the price. if you make an endgame aura tower to boost speed or damage (see green TD for the idea), then you codul make it cost a whole lot, because it would be really useful. Another idea is to have it take up a lot of space (like a tower command center) which would limit the placement of other towers or higher tech buildings. If you did a tower command, it could have various upgrades to towers, or maybe produce "tower pilots" that could garrison individual towers. This could be done by making the pilots items that fit in items slots of the buildings. I don't know how vamp beast does that, but you can put up to three "gems" on each tower that can increase either speed or damage by 5%. Also, the command center could have multiple auras (devotion, endurance, command) which would cost money to research and upgrade.
You could balance an aura tower by changing the magnitude of the buff (maybe lvl 1 would only increase speed by 3-5%) and the area of effect of the aura (only hits the towers right next to it)