The thing is that this is like putting up a lock when you hear a master thief is on the lose. Will the lock stop him? probably not. But will it slow him down maybe by a fraction of a second. the walls aren't there to STOP the invasion they are there to slow it down, besides if you encounter such walls you will need to hire a cannon force/wizard which will take away war money for normal soldiers.(As an after thought couldnt you get enchanters to reinforce the walls?) edit: I know there are some people saying the same thing but I posted this then stated reading the otherones sorry

The thing is that this is like putting up a lock when you hear a master thief is on the loose. Will the lock stop him? Probably not. But will it slow him down? Maybe by a fraction of a second. The walls aren't there to STOP the invasion. They are there to slow it down. Besides, if you encounter such walls, you will need to hire a cannon force/wizard, which will take away war money for normal soldiers.(As an after thought, couldn't you get enchanters to reinforce the walls?) edit: I know there are some people saying the same thing but I posted this, then started reading the other ones. sorry