Remember Nihil, that walls block LoS so if your archers can't see the over the walls then they can't attack over them.
Though, using walls to block a spawn point or bottle-neck certainly is the way to go. Especially when you have put up DOT zones like Poison Cloud to cause their creatures early damage when coming out of the spawn point, and depending on where they placed the spawn point, you could completely isolate the creatures coming from the zone. A good tactic that I have developed to deal with those points in to play a Poison Cloud, and throw in two or three Walls of Fire, depending on placement of the spawn point, so that the walls can't be hurt by physical attacks and i have a forced path for walkers to take. The DOT damage makes killing a lot easier, and the loss of the "sprint" action keeps swarmers from running creatures out of their spawn points. Should they try to run through the fire I have a Force Push waiting to send them back. Sure, I have to spend 3 mana to send them back through, but that equals out to 10 attack dice worth of damage at a max of 20 regular damage (if you perfect rolled reg. dmg) or 20 crit damage (again if you perfect rolled) and 2 - 4 burn tokens.
If a swarmer tries to run through that, their creatures would seriously be in pain.