I personally hate the graphic design of Balastor’s Barracks – that old school dungeon intended to fill up a single page – but I do think there is something LIKE it at that location. An underground complex, built as a bunker-barracks and a minor temple to Humakt, no doubt existed near the Griffin Gate.
Nowadays it is thought to be haunted by ghosts and undead. A giant is often seen in the area, and there are more than the usual number of broo seen in the Griffin Gate area. In other words, I’d keep much of the inhabitants and challenges there.
But I would completely redo the map. And borrow ideas from both Derinkuyu and WW2 Berlin. Tie the result into Glorantha and make it make sense!
And whatever results doesn’t need to fill a rectangular page!
Among the biggest residential buildings I am aware of in the Big Rubble is the city-block sized, three story building now used by the Krang Clan as a stronghold. I imagine there might be a few more buildings like that in Trolltown, also each the stronghold of a troll clan (and probably more powerful than Krang’s Clan).
BUT there are limits to this. First while there are city block sized buildings built around a central courtyard, they are made of stone, not brick. And I doubt many were more than three stories tall (as opposed to the nine story high insulae of Ancient Rome).
The ruins around Griffin Gate are nearly all with broken walls, partial roofs and ceilings, and largely uninhabitable in any case.
Something like Derinkuyu is a much better inspiration for the underground networks in the Big Rubble than the classic AD&D “one full page” map: