Preserves are safe havens where most magical creatures dwell. They are run by mortal caretakers who keep everything in order as best they can.
Magical boundaries are in place to prevent some creatures from going to places outside their territory. This is both for the safety of other creatures and the caretaker. The gates of these preserves are usually hidden from most mortals by distracter spells or some other similar form of magic.
In many preserves, truly evil creatures such as demons are imprisoned and bound with spells.