The zombies are unable to enter the red team spawn in any way..
The problem with this is that survivors can camp inside behind walls and wait the timer out (which makes them lose anyways) but it is a waste of time nontheless.
A lot of rounds go on for a lot of extra time of just waiting for the timer to run out (and chucking guillotines) because people won't come out of the spawn (to die horribly).