The closest is Silesian Ostrava Castle, 1 km south-east of the city centre; the most famous is Orava Castle — #234 of 8,133 worldwide — 100 km south-east. The oldest dated site in the circle is Zamek Piastów Kozielskich, founded 1222.
The 100 km circle reaches into 3 countries — Poland (32), Czech Republic (19), Slovakia (13).