Half true, so to speak. It is true that Josef Menčík was an owner of the castle where he lived. This castle was Dobrš, possibly built somewhere in the mid-14th century, with the first preserved mention about it being written in 1377. Initially held by Koc family until 1630s, when the castle has changed hands frequently until it has been bought in 1707 by Adam Schwarzenberg who renovated it in the Baroque style, with a ruined tower being an only remnant of the medieval castle complex (you can see its present state here). His family held this residence until 1911 when, after several storms that damaged the roofs, the entire building got severely damaged by fire. Ruined palace was bought for about 1000 Czech crowns by Josef Menčík, a 40-years old descendant of an old local noble family. Being a great afficionado of history, especially the Medieval history of Bohemia, he attempted to restore the building. After some time, he managed to make it more or less suitable for habitation, and then began to build a collection of historical objects and curios, including old organ from the church at nearby Rabi and a substantial collection of medieval and early modern weapons.
Josef Menčík and his family also lived in castle that lacked many modern amenities, such as running water or electricity, so that the life inside was not as different from the medieval realities, although, truth be told, there were still villages in Central and Eastern Europe that were modernized only well after the Second World War. In his spare time, Menčík was often teaching people about history and held school trips in his castle. Although initially local villagers didn't like the idea that the ruins used by them as a convenient source of stone has been bought and thus made unavailable, they eventually came to like their new, eccentric neighbour.
Did he fight the German army on horseback? Not really. It is true, however, that in 1938, when the German forces entered western Czechoslovakia, securing the freshly annexed 'Sudentenland' (Dobrš was located roughly 30 km [19 miles] from Czech-German border), Menčík, then 68 years old, rode to the the border town of Bučina in full armour, spear in hand, and stood in the middle of the road halting the armoured column, but eventually was forced to desist, with German soldiers pretty much ignoring this modern incarnation of Don Quixote, with some sources suggesting that they did it with a mix of amusement and respect for the courage of the eccentric man. Josef Menčík continued to live in his castle, but due to deteriorating health, he had to move to his son's home. He survived the war, but died on 19th November 1945, few days after his residence was nationalized by the new, communist government.
Pecka, J., Dobrš a rytíř Menšík [Dobrš and Knight Menšík], in: Vítaný host, spring 2011.
