Im making a Maul (Tower Defense), Simple question about blocking

[RiskYourUnits]

Ok im making a maul wich is a tower defense that players can build freely on the monsters running course. so my question is how would i make it so that if the monsters path is blocked by a players tower (on purpose to where they cant go any farther on there path) then the monster will attack a players tower, but it can only attack it if the path is blocked.

 

[Ezekiel12]

Well Im no expert in tower defense but my first idea is to let an invisible unit check the path before the actual spawn. If it can not pass the actual spawn attacks.

Or register for every unit the distance it moved from time to time, if the distance is very small it means it is blocked so order it to attack something.

There might be an easier way though..

 

[coolyoshi1]

well you dont really need an invisible unit. you can just add attacks to all the creeps