Gnarlstat

The abandoned dwarves' fortifications of Gnarlstat can be found at the X below:

The inside of Gnarlstat is HUGE, so huge that I have turned the mini-map of the interior into a thumbnail (click to enlarge):

What makes Gnarlstat so popular is mainly its size, also Dark Elves give pretty good experience. Mostly though, you can just wander in large circles and by the time you reach where you began the enemies will have re-spawned. There are also two other levels,  a mine and a basement area where Acutami of the Swamps is located. Most people prefer to stay on the main floor however.

Here is a few examples of the enemies you will find: