The closest is Forte da Ínsua, 18 km north of the city centre; the most famous is Castle of Guimarães — #636 of 9,079 worldwide — 53 km south-east. The oldest dated site in the circle is Citânia de Sanfins, founded c. 450 BC.
The 100 km circle reaches into 2 countries — Portugal (27), Spain (12).