Pretty good map and interesting idea reminds me of playing warcraft 3 tower defense mods. I have some suggestions though.
1. Perhaps add a 5 chance system where 4 zombies can touch the portal without ending the game, and the 5th would end it? (not all at once, but when one goes in, and then another, that counts as 2)
2. A sort of barrier shield that blocks the zombies from coming into the base for like 15 secs?
3. Add a buyable that shifts the zombies focus to the player(s) for like 15 secs instead of the portal? this would allow the player to draw them away from the portal for a short time, but when the time is up the zombies would revert back to the portal. Without jug and the somewhat tightish corridors it still would be difficult, and also when the 15 secs run out, the zombies at the back of the pack would easily get access to the portal. This mechanic would allow for some interesting gameplay.
I think also maybe coming up with your own perks would be a good idea and repurpose other normal zombie mode mechanics to make your mod more unique. A little bit more distance between the portal and the entrance of the zombies would be good too.
There were a few other things that could be improved possibly, but they were not crucial (mostly choices in the map design, and the position of things).