As you can see in the picture, there is a simple tactic that provides a way to make impossible any conquest. As soon as you demolish the tower, you just put a new one, within the same boundaries. This technique can be done indefinitely, and it breaks the game mechanics, so you become almost invulnerable, which is not correct.
My suggestion is the following: Option 1 - to make possible the attack tower which is in the process of construction. Option 2 - prohibit the construction of a new tower, within the boundaries of the tower which is under attack at the moment.