Holdings I've seen in play:
Alison's Alley - started as an alleyway with a secured gate, spread out to include the building block around the alley as reinforcements (human and material) became available.
Foster's Place - a smallish cement warehouse-type space under an multi-level overpass.
Marie's Other Room - a door in a wall to a weird room that didn't fully exist, where Marie's 'people' lived.
Hillview Mall - a shopping mall full of weird, with one store being the actual hold and the rest being just...weird. Hocus-land, for reals.
The Luxor - A big hotel containing a marketplace on the ground level, rooms for hold-folk on lower levels, offices and infirmary and etc on the mid-levels.
Also, one could make an argument for The Garden and Wensdy's Factory as holdings, which are secured sets of rooms in different places in the Luxor during a very tenuous and unsettled time.
I think a holding is not necessarily a keep, but it does often play like that.