You cannot access Bthardamz unless you are on a mission for the daedric lord Peryite. The shrine of Peryite is east of Bthardamz, and a little south. Once you are on the mission, you'll be able to get in, the vines will disappear.
If you are indeed on the mission and you still can't get into Bthardamz, then you've got a serious bug and may have to wait for a patch.
You can only access Bthardamz if you've started Peryite's daedric quest, issued to you if you visit his shrine.
You have to have the required woodcutting level and a hatchet
You need to be Lv20 to start the 'Pieces of the Past' quest.
It is at the far left past the living quarters. It is covered by vines that have to be removed.
You don't kill him, you have to make him chase you into the keep (right behind you.)
Yes, judging by the past GOTY Editions of the past games (Oblivion and Morrowind), the Skyrim GOTY will feature ALL the expansions, as that is what a Game of the Year Edition is: The game with ALL expansions. To give you an idea of how what a GOTY is, please visit these links.
You use your slingshot
Skyrim is a 15.
Das Skyrim.
liana vines
vines
Vines