Heartwood

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

Lore
''The Elfs live high in the branches of a vast tree, known as the world tree. The city of Heartwood exists outside of Sosaria, yet the roots of the tree have a magical connection with the land of Sosaria. This mystical tree is simultaneously experiencing all seasons, a different season in each quarter of its girth. Where the Elfs have made their homes, it is always early spring. The Heartwood is extremely different from other Sosarian cities, because Elfs approach life differently than Humans.''

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.