Vulnerable physical door the Airship Portal adds to a saferoom's exterior roof; destroying it grounds the hangared airship.
The Airship Portal is not free of risk. Installing it makes the system add what amounts to a trap door to the ceiling of the guildhall — a real, physical door appearing on the exterior of the designated Saferoom's roof. Unlike the saferoom itself, this doorway can be damaged, just like the magic doors to the stairwells on the previous floor.
If it is destroyed while an airship sits in the hangar, the ship is unavailable until repairs are made: the equivalent of losing a runway at an airfield. People trapped inside the hangar can still exit via the elevator so long as the guild hall stands. Florin's insistence on rushing base defenses reads directly against this weakness.
A physical door set into the exterior roof of the designated saferoom.
The Airship Portal's structural weak point — a destructible runway the enemy can take out.