Kind of repetitive

First time playing but on survival mode after filling up the whole map and killing what all four kind of guys.. It gets a little boring and repetitive. I feel like Ive been playing whakamole for the past 30 minutes only the moles turn into zombie whales.

So Ive filled up most of the map with the same 5 buildings and the only thing changing is the number of guys now.. Is this it? I quit after half an hour. I just got bored. :(

Antiscam223 about BioDefense: Zombie Outbreak, v3.1