If you want to write your script yourself,you can probably make a timer.
if player in zone then
starttimer
if timer = something then
activate if used
end
end
You step on a trigger, it spawns how ever many zombies
when however many zombies are spawned and killed spawn another trigger
do over how ever many levels
making the ai (zombies break the windows, and walls and then go after the player) is not something I can do, but the rounds should be possible.
most of this can be done with triggers with the propper name, the timer in between rounds will need a script but if the time between rounds is the same you could run the same script over and over. Learning lua is worth the time and effort if your serious about making a game with GG. I will work on this and see what i can do but im not smallg.
Markchapman10 is my Skype let's have some dev talk.