Where did Mark Twain do most of his writing

In the Billiards room, oddly enough.

Billards room

Billiards room

Upstairs billiard room.

Yup, He had two writing desks. A nice big one that faced the billiards table (which he was often distracted by) and a small one next to it that faced a wall- which was for "serious writing". Haha.

Billiard Room

The room upstairs. It was the billiards room.

Much better than expected

Billiard room, upstairs.

That's correct. The tour was wonderful