The closest is Olomouc Fortress, 1 km east of the city centre; the most famous is Špilberk Castle — #401 of 8,133 worldwide — 65 km south-west. The oldest dated site in the circle is Trenčín Castle, founded c. 1050.
The 100 km circle reaches into 3 countries — Czech Republic (47), Poland (12), Slovakia (5).