This feature will be a great addition and can add more depth to gameplay if you can add tower abilities. For example, the tower Cannon will shoot a giant cannonball on an enemy and deals massive damage on a small area. So, these kinds of abilities can only be done on a set amount of time and has a cooldown before casting it again. Of course this could be upgraded in the Research section to reduce its cooldown, increase the ability damage, increase AoE, etc. Adding this would be an epic update to the game and it will be more enjoyable to play.