A simple tower that supports your defense. This tower simply just shoots spells at attacking enemies, but these spells are significantly weaker than even their level one counterparts. It would shoot all the spells at attackers except for jump spells (for hopefully obvious reasons). Upgrading would bring the effectiveness closer to a level one spell, until max level it would hurl level one spells.
This sounds great, right? Here's the catch, It is absurdly expensive to reload the tower (I was thinking ~100 k of elixir after each defense).