I guess the observatory would be a good place to have survivors providing somebody finds a way to get all those barricades in the 'stronghold assault' mission to spawn. (Granted, this probably won't be possible until Madman releases the script source for either that mission or
Defender of the Castle.)
The asylum is well fenced off from the rest of the world, but I'd be skeptical about using that since that's where all the crazies came from the first place