Yes, from scratch!
Zombie Torrent 2's wave system goes back years into the past, starting with an obscure mod called Imp Attack. Taking this script and building upon it, the wave system made its way into Zombie Torrent. Today, the system has been touched up on even more and exists in Zombie Torrent 2.
What does ZT2's invasion system have the ability to do?
- Execute scripts on certain waves, ala invasion
- Halt between waves, giving players time to shop
- Disable itself on "sandbox" maps (testing maps)
- Runs in the Cooperative / Survival gamemodes
- Operates on a pool count, rather than spawnspots
- Uses a unique playlist system for randomizing music
- Clamps zombies on-map to a certain CVAR value
- Uses a special last-wave script system for a boss
In a sense, you could consider ZT2's wave system "invasion done right" in more ways than one. It is highly flexible in the sense that it offers configurable break-time between waves. Not only that, but it uses a custom UI rather than a hardcoded gamemode interface. Variety is fun!