Olsene Castle

Belgium

Olszanica

Poland

Olsztyn Castle

Poland

Olsztynek

Poland

Olteni

Romania

Olyka Castle

Ukraine

Omi Hachiman Castle

Japan

Ommerstein Castle

Belgium

Onomichi Castle

Japan

Onsenoort Castle

the Netherlands

Ooidonk Castle

Belgium

Ooigem Castle

Belgium

Oorbeek Castle

Belgium

Oostkerke Castle

Belgium

Opeka Manor

Croatia

Ophemert Castle

the Netherlands

Opheylissem Castle

Belgium

Opleeuw Castle

Belgium

Opoczno

Poland

Opole

Poland

Opole Lubelskie

Poland

Oponice Castle

Slovakia

Oporów Castle

Poland

Opprebais Castle

Belgium

Oradea

Romania

Orago

Italy

Oramala

Italy

Orava Castle

Slovakia

Orchomenus (Boeotia) Castle

Greece

Ancient period

Ordingen Castle

Belgium

Ordubad Castle

Azerbaijan

Oreshek Fortress

Russia

Oria Castle

Italy

Orino

Italy

Orneta

Poland

Oroszlán Castle

Hungary