2 ways.
1: Place a brick in the center of the tower. Give it these events:
OnActivate<Self<SpawnExplosion<Rocket [ -------------2 ]
Although it won't be as big as the following.
First, get the name wand. Then set the name of the baseplate of the tower as Boom. Then hit it with the namewand. After that, make a brick with these events.
OnActivate<NamedBrick[Boom]<SpawnExplosion<Rocket [ ----.5------------]
Note, it may be laggy.