All my Survivors were defeated and the screen says Game Over! Is that all there is?

Not at all! The unique thing about Survival City is you get to try again, even after a game over. You will start with 2 Survivors again but all the weapon upgrades you have carried out will carry over. All the buildings you leveled up with carry over too, though some may need repairs. The layout of your City will be the same. 

Starting with these level ups means you can strategize and try to survive more nights the next time around!