If, when the Horde attack, the town is deserted (i.e. all surviving citizens are camping outside the walls OR have been killed in town), it will be considered devastated by the zombies :
- the gates are irreparably damaged: the town will no longer be able to serve as a refuge for the survivors,
- buildings under construction will have to be started again from scratch,
- the water reserves in the well will be affected,
- it is no longer possible to carry out upgrades,
- citizens' houses are destroyed (interior improvements and upgrades are unaffected),
- Chaos mode is immediately in effect (even if the usual conditions are not satisfied).
Your days are numbered... A little advice for such a situation : Get the hell outta here!