Complex sentence: A powder magazine capable of storing 350 barrels of gunpowder was constructed underneath and the tower could provide accommodation for a small military detachment of 1 officer and 18 men, with stores for the battery.
Split into simpler sentences: A powder magazine capable of storing 350 barrels of gunpowder was constructed underneath. Accommodation for a small military detachment (1 officer and 18 men) with stores for the battery.