The latest patch reworks how magic functions.
Magic is now purchased and upgraded from the coven, but is no longer consumeable. It works on a cooldown basis now. This is in response to the fact that players weren't often buying them as they took away from the resources required for towers on a level.
The spells themselves have been rebalanced also, the tier 3 spells are now much more powerful and tier 1 spells are much weaker. This better represents their cost and cooldown.
There has also been some bug fixes in the latest patch along with some UI changes.