Castelo de São Jorge: (St. George's Castle), (Walk up the hill from Alfama or take bus 37). 9AM - 9PM (March - Oct) and 9AM-6PM (Nov-Feb). Located up the hill, with a great view over the city and the river. If you have the energy, get there by walking from downtown, going through the fantastic old neighborhood of Alfama. ?7 with student discount available.
Ponte 25 de Abril: This sister bridge of the San Francisco - Oakland Bay Bridge was designed by the same architect in 1966 to connect Lisbon with the Setubal peninsula across the Tagus (Tejo) River. Formerly known as the Salazar Bridge, it was renamed after the Carnation Revolution, which on April 25, 1974 ended the dictatorship! edit
Ponte Vasco da Gama: It is the longest bridge in Europe (including viaducts), and ninth longest in the world, with a total length of 17.2 km (10.7 mi), including 0.829 km (0.5 mi) for the main bridge, 11.5 km (7.1 mi) in viaducts, and 4.8 km (3.0 mi) in dedicated access roads.
Aqueduto das Aguas Livres: This is a historic aqueduct in the city of Lisbon, Portugal. It is one of the most remarkable examples of 18th-century Portuguese engineering, including the largest stone arch in the world. The main course of the aqueduct covers 18 km, but the whole network of canals extends through nearly 58 km. The Mãe d'Água (Mother of the Water) reservoir of the Amoreiras, the largest of the water reservoirs, was finished in 1834. This reservoir, with a capacity of 5,500 m³ of water, was designed by Carlos Mardel. It is now deactivated and can be visited as part of the Museu da Água (Water Museum).