LOL. That would have been interesting!
I did figure out that it can run on its own and upgrades can be delayed, but not too, too long or they'll eat the scarecrow. Even longer and they manage to get one resource out of the castle, but then die quickly after hurriedly adding turrets and upgrading...

On this computer (i5 9500 2.2Ghz, running Ubuntu 20.04 Desktop) and Chrome, each game is between 30 and 40 minutes. Completely acceptable since it can run in the background.
I now have both Avatars and both achievements. Not planning on playing this any more, unless there is a solid reason to do so.