According to a nearby information board:
When excavation of the Guard House was completed in 1969, the site was reburied to protect the feature. Therefore, little is visible today. The Guard House was divided into two rooms that reflected its dual uses. The southeastern room was where soldiers on guard duty were stationed. The northwestern room was used to house prisoners. It had a floor of mortar poured directly on the bedrock, presumably to keep prisoners from escaping through the floor. There is no evidence of bars or any other means of physically confining prisoners, so it is likely that the only access to the prisoners’ room was through the other room where the guard was stationed.
A number of courts-martial were held at Fort Montgomery. Soldiers were usually confined in the Guard House until their appearance before the court. Many soldiers brought before the court were charge with drunken behavior or failing to report for roll call. Abuse of officers, letting a prisoner escape, and a murder were among the more serious charges. Punishment ranged from small fines and public reprimands to demotions or public floggings.