Wind

In Ultima Online, the town of Wind in Britannia is located deep in the insides of a mountain range and accessed over a teleporter north of Britain.

Description
Since the town is inhabited by mages who have no wish to allow mundanes entry, only a mage can use the teleporter. The town is a result of the forces creating this version of Britannia, as it doesn't exist in the main reality. Entering, one has to first overcome the nasties in the tunnels, before entering the city proper, which is protected by magic. The city itself is a very beautiful place, and filled with shops which offer the best goods and especially magical items and reagents. It however is not advised to enter the park, as it is overrun by monsters.

Trivia

 * To use the teleporter, a magery skill of at least over 60 is needed, otherwise it won't work.
 * The nasties in the tunnels include Liches and Daemons, making one wonder how the mages manage to hold these hellish and powerful forces back.