Thanks for commenting on it. This map is not mine and I have never played it. So I have investigated how it was done. The final door will open once 60 zombies have spawned. The problem I found was that depending on how the players are positioned, more, less or no zombies will appear. My recommendation is to group up and look for a place in the water that allows the zombies to spawn constantly since the zombies do not appear in places where you can see them spawn. This should not take more than a minute. When the door opens a message will appear so it is not necessary to watch it.

End of the map broken - door is bugged or doesn't open