The Castle of Dux today

Dux (German name) is now Duchcov in the Czech Republic, near the German border. Casanova was librarian there for 13 years until his death in 1798. This is a portrait of Casanova in the library...

Not really. That's The Bookworm by German painter Carl Spitzweg (1850).

The castle was rebuilt in the 19th century in a Neoclassical style. The Waldstein family who owned it left in 1921 and, at present, the chateau is state-owned and open to visitors. This is a rear view of the castle and gardens.