The Moorish citadel
Before 1147 this was the alcáçova, the fortified heart of Muslim Lisbon, until Afonso Henriques took the city with the help of crusaders.
From the highest hill in the old city, the castle looks over Alfama, the Baixa and the Tagus. Here's what you're looking at, and what to listen for when you get there.

Before 1147 this was the alcáçova, the fortified heart of Muslim Lisbon, until Afonso Henriques took the city with the help of crusaders.
The kings of Portugal lived here until the early 16th century, when Manuel I moved down to a new palace by the river.
Much of what you see was reconstructed in the 1930s and 40s, after centuries as barracks, prison and arsenal.
The hill has been fortified since at least the Iron Age; the walls you see grew from the Moorish citadel and were heavily restored in the 1930s and 40s.
Yes, it was a royal residence from the 13th century until the early 16th century.
In 1147, when Afonso Henriques captured the city with the help of crusaders on their way to the Holy Land.