Zombies keep walking through walls

[RebornArmy]

I have been playing since launch and I've never had walls stop zombies during a defense. Is it something i'm doing wrong? I've had to build on stilts because zombies keep walking through all my walls on ground floor. Tried stairs when built on an elevated platform too and they just attack from inside foundations and cant be attacked.

[BurlsoL]

Sometimes if walls are too close where enemies will spawn in, or were too recently placed, enemies will walk through them.