Located above the sacristy of the church of Nossa Senhora da Anunciação (Our Lady of Annunciation). – in its turn located inside the castle - it is accessed by flights of stairs outside that church.
Of Manueline style, the space was probably built in the 16^th century, although originally it may have had another function before being turned into a chapel. It had a confraternity of the same invocation, reason why the space may first have been the headquarters of the same, already serving for the cult of Santo António (Saint Anthony) in 1758. It had its own belfry with a small bell of bronze.
It has a coverture of sharp edge ribbing and circular granite endings. João António Silveiro e Silva ordered its neoclassical style painting in 1828, according to the existing inscription in that space.
The altar is lined with marbled stuccos, where St. Anthony is worshiped.
 

REFERENCES
ARQUIVO NACIONAL/TORRE DO TOMBO, Memórias Paroquiais, Vol. 39, nº 150, pp. 891-910. Disponível on-line em http://portugal1758.di.uevora.pt/index.php/lista-memorias/78-viana-do-alentejo/3420-viana-do-alentejo-viana-do-alentejo. 
ESPANCA, Túlio, Inventário Artístico de Portugal: Distrito de Évora: Concelhos de Alandroal, Borba, Mourão, Portel, Redondo, Reguengos de Monsaraz, Viana do Alentejo e Vila Viçosa, Lisboa, Academia Nacional de Belas-Artes, 1978,Tomo IX, Vol. 1.