Given that text indicates that Demonreach's prisoners include "Dark Gods", I'm willing to entertain the possibility that "mere" Titans could be imprisoned there.
Also, that the summoning circle exists for an ascension ritual for in extremis emergencies for the Warden.
I found myself wondering how that would work. The one ascension ritual that Harry knows involves necromancy, and absorbing the life of ghosts and other undead spirits. I'm fairly sure there aren't many dead people in the island.
Perhaps there is another ritual that only involves "spirit" power, and I'd guess that either the spirit of the island--or Alfred--would suffice. However, I'd think that sucking the power from either would be counterproductive to the Warden's responsibilities. And I'm pretty sure that Alfred would...quibble.
Then there is the fact that if the Warden
were to ascend to godhood, he'd probably find himself opposed by thirty or forty beings of the same power level. Not to mention all of the "lesser" beings.
I suspect that someone, somewhen here has calculated just how many could be imprisoned every few feet in twelve tunnels a "mile or two" long.
After all, I recall that someone
did make a good estimate of just what Harry's share of the diamonds was actually worth!