Skipped this part before...

Mages in towers would donate mana to their tower. As tower powers are used, the powers would drain the resevoir of mana from the community pool. If not enough power was present, they couldn't use thier cabal powers.

Capture of their item would cause power to drain away quickly. Capture of another tower's item of power would cause their power to drain into your towers.

This would add a sense of urgency to protecting your item, and a reason for the towers to compete with each other.

...Rome...