Colón's main draws lie outside the city centre itself. The Gatún Locks are a short drive away and offer an impressive close-up of canal operations. Fort San Lorenzo, a crumbling Spanish colonial fortification at the mouth of the Chagres River, is one of the finest examples of Spanish military architecture in the Americas. Portobelo, up the coast, is a UNESCO World Heritage Site and worth a half-day. Within the city, the Zona Libre de Colón is the world's second-largest free trade zone — a strange and vast wholesale zone that few leisure travellers enter but which defines the economic life of the city.