I spent the first day of my recent Easter trip to Lisbon walking around Alfama and visiting the Castle of São Jorge. The original castle was built in the 12th century on the site of a Moorish citadel and was further developed in the 16th century. It was, however, destroyed in the 1755 earthquake and the castle we see today is the result of renovation that began in 1938.
Visitors can climb the towers and walk along the castle walls, see the archaeological site just outside the castle or visit the archaeological museum. In Torre de Ulisses there is a camera obscura that projects views of Lisbon onto the inside walls of the tower.