Červená Lhota Castle is a Renaissance château in South Bohemia. It stands on a rocky island in the middle of a small lake. There is a large park around the castle. The picturesque scenery as well as the well preserved interior of the castle is a destination of thousands of tourists every year. People call the castle a red pearl on a lake's island.
However, there is a mystery behind the walls of the romantic castle. Upon one of the walls, there is a cross that cannot be repainted. According to the legend Devil is the author of the cross. He made it using the blood of Johanka, a very pious lady who converted from Catholicism to Protestantism. The castle is now painted red so that people cannot see the cross, however it is still visible. The colour of the walls is the reason why the castle is called Červená Lhota (the Red Mansion).
sembra naturale?

Červená Lhota Castle is a Renaissance château in South Bohemia. It stands on a rocky island in the middle of a small lake. There is a large park around the castle. The picturesque scenery, as well as the well-preserved interior of the castle, is a destination for thousands of tourists every year. People call the castle a red pearl on a lake's island.
However, there is a mystery behind the walls of the romantic castle. Upon one of the walls there is a cross that cannot be repainted. According to the legend the devil is the author of the cross. He made it using the blood of Johanka, a very pious lady who converted from Catholicism to Protestantism. The castle is now painted red so that people cannot see the cross, although it is still visible. The colour of the walls is the reason that the castle is called Červená Lhota (the Red Mansion).

—Very well-written. My suggestions:
“the devil” (impersonal) should not be capitalised; if you give a personal NAME to the devil then you would use Satan.
“Reason that”, not “reason why”: reason means “why” or cause of something. Therefore reason+why is redundant, although it is frequently used in casual speech.