The closest is Špilberk Castle, 1 km west of the city centre; the most famous is Čachtice Castle — #313 of 8,133 worldwide — 99 km south-east. The oldest dated site in the circle is Burg Raabs an der Thaya, founded 926.
The 100 km circle reaches into 3 countries — Czech Republic (46), Slovakia (9), Austria (9).