The shared antechamber outside personal spaces, which becomes a long hallway of doors when many crawlers use one entrance.
When the crawlers exit their guild saferoom into the FUPA via the Bait Shop, the situational space between the entrance and their personal quarters stretches into a long corridor lined with door after door, one for every crawler coming and going. Carl never thought through the implication until it bites: off-worlder Sponsors can hold personal spaces too and can buy the Doggie Door Upgrade, so any of them who ever used that bait shop keeps a door there forever.
During the attack on the base, Bomo notices an extra door in the corridor belonging to The Reavers, and Donut realizes the enemy is running The Bamboozle on them from inside the castle walls. Carl orders Bomo and Sledge to hold that door and not move.
A long hallway of identical saferoom doors set one after another, growing longer as more crawlers use the same entrance.
The infiltration vector that lets the Reaver team walk into the crawler stronghold.