Ponte da Barca

Overlooking Spain, in a dominant position in the Amarela Mountain this castle was built from scratch in the Middle Ages, to defend and protect the country borders.

It is considered as one of the most important Portuguese military monuments, for its technical and architectural innovations.

Some authors affirm that the place name derives from the Latin Lindoso "Limitosum" (limiter, border, extreme). Although there is no information about earlier human settlement of this area, this name is not yet mentioned in the Inquisições of 1220, appearing only in those of 1258. It is believed the fortress was started in the 13th century under the reign of Afonso III, as an effort to strengthen the defensive system of borders. The castle was reinforced and enlarged in the reign of King D. Dinis, in 1278.

http://alfarrabio.di.uminho.pt/lindoso/castelo.htm
https://pt.wikipedia.org/wiki/Castelo_de_Lindoso