Heartwood

In Ultima Online, the town of Heartwood in Britannia lies a little south of Yew

Description
The city can only be accessed through a portal located in the center of the town of Yew. It is the home city of the Elves and built into an enormous tree. The town is a result of the forces creating this version of Britannia, as it and the race of Elves don't exist in the main reality. Just like a human city, Elves of all professions and backgrounds are found, most being neutral or somewhat friendly towards humanity. However, most Elves aren’t interested in gold and want to trade what they have for something they want in barter. The city is also the source of many recipes for wondrous elfish items.

Trivia

 * The concept of an elven city in a giant tree is not a new concept. It was used, for example, in Baldur's Gate II as well.