As for aggro, I am not certain. Big boy was inside the building when I got to the base and thralls did not seem agitated. All building doors were closed
From what I can tell, he came up one of 2 stairs (see pictures).
I never had an animal inside the base while there were just walls and closed doors. I opened the walls with those stairs only few weeks ago. So I conclude that there was no spawn inside the base.
On one previous similar encounter a mammoth was inside foundations at the top of the stairs. Like standing on foundations 1 or 2 levels lower and ignoring the top ones. When aggroed, he almost instantly clipped to (proper) top level of foundations, which also triggered NPCs to react.
From what I can tell only by short observations, when a big animal gets in narrow places and turns around, the walls/foundations it touches, are then ignored by the body.
Here are some screenshots of the possible approach points, the house in the base and the narrow corridor leading from stairs to the house (with coordinates):
If you need more info, please say.
EDIT: Here is a video of me pulling a mammoth into my base (same location). Few very obvious instances of mammoth walking on lower level foundations and ignoring top ones.