Just east of the Ruins of the Church of St Paul, Monte Fort was built by the Jesuits between 1617 and 1626 as part of the College of the Mother of God. Barracks and storehouses were designed to allow the fort to survive a two-year siege, but the cannons were fired only once, during the aborted attempt by the Dutch to invade Macau in 1622. Now the cannons on the south side are trained at the gaudy Grand Lisboa Casino like an accusing finger.

On the outside of the southeastern wall, about 6m from the ground, under a cannon, is a sealed rectangular opening. This was a door used by soldiers patrolling the old city wall, which was connected to the fort at a right angle.