The question is how to make the mobs attack the player's towers when you block their path. The answer is that the mobs should do so on their own if they're blocked as long as there isn't a different way to their destination. You may want to add a trigger that, each time a creep attacks, executes a 1-second wait then reorders the creep to move to its destination (so once a path is open the creeps don't keep attacking anyway).
(Already solved) Another doubt: I made my builder fly, but he doesn't builds, now: The building appears with 100 HP and stops! Why?
"It's just change the builder's race to Undead - so he will let the building 'making' itself. Or Orc - so he will 'enter' the building to build it, like Night Elf"
Some people can also have this question, so I'll leave a possible answer here.