Could've sworn I replied to you. In any case, go to Destructible in the Terrain Module's Units Palette. Looking for "Tower" under the Destructible category will give you all collapsible terrain objects. Triggers are unnecessary unless you want to do something with the resulting debris, such as make a checkpoint for a campaign that blocks the player from going backwards. It's an effective way to push progress forward. A trigger for this is outlined below:
Event: Unit Enters Checkpoint1
Conditions: Trigger Execution Count == 0
Actions: Kill (Rock Tower)
(Send Transmission if you want a message to play afterwards)
If you want the resulting debris from the Tower to be indestructible you could go into the Data Module and find the resulting debris "unit" attached to the Rock Tower that your trigger kills. Go to its Flags and fill the checkbox next to Invulnerable.