Originally posted by FinalAnswer
It was in Ostrheinsburg Castle.
"Eventually, Siegfried and the resurrected Azure Knight would meet again at Lost Cathedral. Zasalamel used his dark arts to break Soul Embrace and both the Spirit Sword and the Cursed Sword were freed. Both Siegfried and Nightmare took their repective swords and clashed one more time, causing and explosion that destroyed the entire cathedral and nearly destroyed them both... "
That cathedral was huge last I recall, pretty much was a castle.
Squall is a non-factor here, he has not the showings to put him on the same level as anyone here.
Durability? The class that destroyed the cathedral/castle did not even harm Nightmare at his weakest, Soul Calibur can heal Nightmare from death, and Algol exists as long as he wills himself to.
Algol was creating cataclysms across the planet, hurricanes and the like.
Sephiroth is really the only real threat on team 1, though honestly with his superior speed and telekinetic powers, he may solo.