If you really need to store the information, you can use GUI or local variables with jass. To do it in GUI, make a variable array with a really big index, then set the variable to the tower type of the destroyed tower to store the tower type information. After every execution of the trigger, set the variable index (the index will be a variable) = to itself + 1.
Then after every timer confirmer, reset the index to 0.
The index will be the number of towers destroyed and the variable Tower_destroyed(index) or Tower_destroyed(1) if index > 1, for example if you had a variable array called "Tower_destroyed" would be the tower type. If you had a variable array called "Point", then Point(1) to Point(index) would be the point where each destroyed tower was destroyed.