This photo of the Benicia Arsenal Guard and Engine House, erected in 1872, shows the structure as it was originally built:  a handsome composition of brick trimmed in sandstone, later covered with stucco. Although the reason for combining these two different functions remains obscure—possibly budgetary considerations—it incorporated both a large space to serve as the engine area and also an area with a number of small cells. Each functional space was configured to be self- contained with a separate, designated entrance for each. The guardhouse portion contained a prison with two solitary cells, and also included an office and quarters for the guard. As these interior prison cells had little light or air, the ventilation system was designed to incorporate a flue from each cell, which vented from the rafter space into a large fireplace stack. When the fire was lit, the air was circulated by the movement of air being drawn out of the chimney and replaced with air from the outside.