...a carronade! From the marker next to this model in the Casemate Museum:

Carronades were naval weapons, but they were also used by the Army at various
seacoast fortifications when the Army's equivalent piece, the 24-pounder
flank howitzer, was not available. Although it cannot be confirmed, it is possible that
some carronades were mounted at Fort Monroe. These guns, or similar armament, would
have been used against enemy troops trying to attack the fort along a front.
Such guns were also placed to protect a fort's entrances.

By looking through the embrasure you can see how such a piece would dominate the moat.
Since these guns were normally placed in casemates on the shoulders of a bastion,
they would nhave been highly effective in clearing the moat of any attacking troops.

This model was built around 1880 as a training device.