Only transfer, prime-number and stairwell stations have hatches in the floor — which is why the boring stations are now the safe ones.
Carl's chat losses come out at roughly twenty percent, devastating but much lower than he anticipated, and the reason is a piece of level design. The Krakaren Juvenile Cloneling invaded all the transfer stations, but the regular, empty stations that are not prime numbers and not stairwell stations have no trap doors in them — and those places all have only a single entrance. That makes them defensible: Daniel Bautista's group holds stations 64, 65 and 66 from the tops of thin stairways, and groups all around the tangle are doing the same.
The corollary is that the interesting stations, the ones with crystals and stairwells, are the ones with a hole in the floor and a swarm underneath it.
The distinction between which stations can be held and which cannot.