WorldGuard Chunk Flag adds a flag to WorldGuard: chunk-unload. If set to deny the chunks that intersect with this region are not unloaded when no player is near them. There are also autimatically loaded when the server starts.
/region flag city chunk-unload deny
keeps the chunks of the region "city" loaded.
/region flag city chunk-unload allow
resets the value and allows the chunks to be unloaded again.