Predating Portuguese sovereignty, the castle of Montemor-o-Velho is a listed National Monument. Perched on a hilltop overlooking the flood plains of the Mondego the castle still dominates the skyline. There has been a fort or castle of some sort here presiding over and protecting the Mondego valley from as long ago as 2000 BC.
The imposing castle we see today dates back to the 13th century with some 16th century additions. Besides the walls and towers there isn't really that much still standing today. There are the ruins of the Paço das Infantas Palace, a favoured royal residence in the 14th century where it is said King D. Afonso IV plotted the fate of his son's mistress, Ines de Castro. There is also the small church of Santa María de Alcáçova, reconstructed in the 16th century Manueline style apparently by Diogo de Boitaca (of Jeronimos Monastery fame).