The Haunted Forest : My first map

i'm making a map,where you choose your hero and you have to get back the precious item of The king which he lost in the forest,I made : 30% ( First version will be in french )
For the moment :
2 Quests
3 Heros ( Druid , Barbarian , Mage )
14 Spells
I'm making the third Quest its a mini tower defence and if someone could tell me how can i make Monsties kill towers if it lock the Road