This is not just with towers people. Any object which is detonated or destroyed will still remain there except invisible. This is due to the "collision" feature. If the object can be collided with while it is visible, it will still be capable of being collided with after being destroyed.
The un-destroyed object you see is picture #1. After picture #1 is destroyed, it's alpha changes to 0% (invisible), but it is still there. Then Picture #2 (The picture of the object destroyed) becomes visible becuase it's alpha changed to 100%.