i always thought the yetis built it and enimes came in and destroyed every thing and that the yetis were not strong enough to defeat them because the wife was sick the weapons and stuff were just decorations
As to who built it, I have no idea. I'm not going to come up with some radical theory, but I'll instead work off what little evidence was given. With that out of the way, I assume that Ashei and her family might have had something to do with it, or were residents there at some point. If memory serves, she tells us that her father raised her in the mountains and taught her the various ways of combat at a young age. Her father apparently died at some point. I surmise that the mansion either belonged to her dad or some other predecessor at some point in the past. If such is the case, I'd imagine her family was very militant and prepared for war given the mansion's resemblance to a well-armed military fort (bastion).
If I had to come up with a radical theory, however, I'd say that whoever the residents of the bastion were, were probably trying to protect their home from the Yetis, but decided to retreat at some point (for whatever reason). [Some of] The Yetis would then take up residence there. The problem is with this is Yeto and Yeta both seem quite nice.. :/ Hmm... Whatever the case, the Yetis were obviously not the original residents. Heck, the entrance was too small for Yeto to enter anyway. lol
Ashei and her father probably lived there, because why else would she have been up there in the first place, unless everyone else was too scared to go up to the snow. There are suites of armor that are human sized, not yeti sized. The Ruins have cannons, so I am suspecting it was built for someone important, or as a protection for Zora's Domain since it sits on the path to Zora's Domain. It could have also been used for soldiers, hence the church and other rooms. The Ruins also have pictures of Ordon Ranch, Hyrule Castle, Arbiters Grounds, the Sacred Grove, and the City in the Sky. This is never explained
"I grew up in the mountains with my father, who was a knight in his own right. He taught me the arts of war as though I was his son." — Ashei (Twilight Princess)
"Of course, lessons in common courtesy were not part of the regimen, so forgive me if I come off as rude, yeah?" — Ashei (Twilight Princess)
"And here I was, thinking Hyrule was empty of men of valor..." — Ashei (Twilight Princess)
"Listen, sorry I was so nasty the other day. I thought you were like those sorry excuses for Hyrule soldiers...but I thought wrong." — Ashei (Twilight Princess)
"Snowpeak, the mountain to the north of Hyrule... It's unlike any mountain I've ever known. I don't know much yet, but things happen there that happen on no other mountain I know of. ...Evil controls it... As soon as I know something more, I'll tell you. Until then, you should stay away, yeah?" — Ashei (Twilight Princess)
"Actually, there's one more of us...but there's been a disturbing turn of events on the north mountain, so she's gone to check things out. She's a beautiful young lady named Ashei. If you have a chance to meet her, you should definitely take the time to talk to her, OK, honey?" — Telma (Twilight Princess)