Bigger than medium creatures choose any square they occupy to make their attacks from, for the purposes of cover. http://www.d20pfsrd.com/gamemastering/combat#TOC-Cover I choose ground level, straight outside the door, so you have no cover.
Regarding squeezing, that is why there were difficult terrain penalties. The walls cause the Ankylosaurus to miss at most 3 of his 64 squares. It's less than having a little underbrush in the square of a medium creature. If the others agree that it causes squeezing, I'm willing to roll with it, but even at -4 for squeezing (and even at -4 and giving you cover on top of it) the attack still hits.
EDIT: Ankou: he said above that the ceilings are only 1 inch thick, so huge and gargantuan creatures automatically destroy the roofs above/below them just by being there.
ROLLS (ALL Rolls from previous turn reposted for continuity):
07:21, Today: Thraxis rolled 26 using 1d20+9 with rolls of 17. Refl Save (+2 cover).
07:43, Today: Thraxis rolled 30 using 1d20+14 with rolls of 16. Tail attack at Winter Hag.
07:45, Today: Thraxis rolled 30 using 4d6+15 with rolls of 3,5,6,1. Tail damage on winter hag.
This message was last edited by the player at 14:26, Mon 26 Oct 2015.