Note 840 — CANTO XXXIII.
James Romanes Sibbald translation
The under gate, etc.: The word translated made fast (chiavare) may signify either to nail up or to lock. The commentators and chroniclers differ as to whether the door was locked, nailed, or built up. I would suggest that the lower part of the tower was occupied by a guard, and that the captives had not been used to hear the main door locked. Now, when they hear the great key creaking in the lock, they know that the tower is deserted.