Fuerte San Jerónimo, close to the center of Portobelo, was the largest fortress ever built to protect the bay. Some two dozen embrasures with 16 cannons face the mouth of the bay, some exactly where the Spanish troops left them in 1821, the year Panama declared its independence. Beyond the impressive gateway, there's the remains of the officers’ quarters, barracks, guardroom and huge observation terrace over the water.
If you’re short on time, San Jerónimo is the most complete of Portobelo's accessible forts and makes for the best visit.