I don't think it really matters whether towers are cheap or not. If you can't break through a defence, how could it be cheap anyway?
Take a look.
X = Archer/crossbowman
T = Tower (which throws stones)
A = Tower with archers/crossbowmen
The attacker in the north has 12 crossbowmen. The defender in the south has 7 crossbowmen between and next to his towers and 2 towers with crossbowmen in them (lets assume we can stash 2 crossbowmen in a tower). That makes a total of 11.
The enemy is moving forward and gets in range of your towers with archers. Maybe 1 or 2 crossbowmen die before they are close enough to shoot back. But now they are being fired at by the crossbowmen next to the towers as well. The crossbowmen next to the towers will probably shoot first and are likely to kill quite some hostile crossbowmen. The hostile crossbowmen will pick random targets within their range and might not even kill the crossbowmen on the tower, making it more difficult to send a next wave because you still have the same problem.
Imagine what it would be like if there are more towers a little more to the south. If those towers are filled with crossbowmen as well, they too might be able to hit the hostile crossbowmen thanks to their increased range. So you have a second defence line that can shoot at the same time as your frontline! Guess that would be quite a slaughter. You could also replace the regular watchtower by another tower with archers and build regular watchtowers just below the towers in the front (so you have 3 towers with archers in the front, but the enemy still can't use his melee units because your regular watchtowers on the second line defend them).