Miradouro da Senhora do Monte ("Senhora do Monte viewpoint") is one of Lisbon's best viewpoints, overlooking the downtown area, the Saint George's Castle and the Tagus river from afar.
One could argue that Miradouro da Senhora do Monte is underrated as it is not as known / visited as other viewpoints located in Alfama, such as Miradouro das Portas do Sol, Miradouro de Santa Luzia and the Saint George's Castle itself.
The Saint Gens hill, in which this viewpoint is located, is historically relevant as it played an important role in the conquest of the city of Lisbon by the Christians against the Moors back in the 12th century.
Lusideias' recommended next step from here would be another nearby viewpoint: Miradouro da Graça ("Graça viewpoint"), just 10 minutes away by foot.