Fort São Francisco — properly known as the Forte Nossa Senhora do Rosário — is a 17th-century star fort built to defend Chaves after Portugal regained independence from Spain in 1640. The fort's angular bastion design is a good example of the military architecture that was transforming European fortification theory at the time.
Part of the fort complex has been converted into a hotel and conference centre, which means access to the interior is restricted. But the exterior walls and the overall shape of the fortification are visible from the surrounding streets, and the scale of the defensive works gives a clear sense of how seriously the Portuguese took the security of this border town. The grounds around the fort are pleasant to walk.