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Ankheg Cave Feels like Cheating (Respawn Areas)
Once you have sufficient weapon to take them out quickly, you can just sit in their cave and grind for days, they continually respawn so you can easily get +3-4 levels out of it if you're patient enough, and if you're collecting all the heads that's tons of gold too.
Anyways, to make this topic more general, what other areas do you feel are really good for grinding using sleep and or traveling to make monsters spawn?