Age of Hyboria

So Thulsa Doom is residing in his Mountain of Power, then he has already established the Cult of Set, a Cult Movement that worships Snakes and answers to the Snake God Set. So amongst the many things he might be doing here, they might be:

Orgy and/or Feast - Every now and again, one such event is held where the attendants are often drugged out of their minds while they feast and pleasure themselves plus each other. Thulsa Doom is usually just observing these going-ons, but he might decide to join in for once.

Research/Experimenting - Either having others do it on his behalf, or do it himself, depending on what's being looked into. Methods for securing more power for himself? De-Cyphering old Scrolls? New Mixtures of toxins/drug?

Addressing his Followers - He may be leading them out to battle, or it might be a Sermon he holds for his Followers.

Preparing to leave - After all, he would have need to exit to conduct business of his own, and certainly so if its importance were great enough to Warrant his being there.