Spa dating from the Iron Age and the beginning of Romanization, having been built in a very similar way to other fortified constructions with bathing functionality.
It has been classified as a National Monument since 1986.
Source: Gabinete Turismo CM Barcelos
https://www.cm-barcelos.pt/items/balneario-castrejo-da-pena-grande-galegos-santa-maria/
Image source: Gabinete Turismo CM Barcelos