TD: Making Creeps attack towers when path blocked

I'm unsure how to make creeps attack towers when their destination path is blocked. Any ideas? So far they just line up and get slaughtered by the towers.

I've had a look at some TD maps, but I can't seem to figure this one out.

Make sure the creeps have attacks.

Event: Unit - A unit is issued an order with no target
Condition: Owner of (Ordered unit) equal to (Player X/Neutral Hostile/Whatever)
Action: Order (Ordered unit) to attack-move to Center of (Goal Region)

I found an error in one of my triggers which stopped the monsters as soon as they attacked. I needed to include a Wait action.