Treasure of Swamp Castle is the English-dubbed name of the movie Szaffi, a 1985 Hungarian animated film from Pannonia Film Studio, directed by Attila Dargay (the person who also gave Hungarian audiences another beloved cartoon, Vuk The Little Fox). It is based on, and has music from, Johann Strauss II's operetta The Gypsy Baron—which is based on the novel "A cigánybáró" (The Gipsy Baron) 1885 by Mór Jókai.
Aristocrats Are Evil: Baron Loncsár is a Fat Bastard with a spoiled daughter. The Governor is not an aristocrat, but has all intentions to become one.
Arson, Murder, and Jaywalking: The Governor's sidekick Puzzola gives the following list of his "preferred crimes" (at least in the original Hungarian dub):
Broken Aesop: A mild example: the movie is quite Anvilicious on that the real treasure is not gold but love... and yet the protagonists get the gold in the end.
Buried Treasure: The Governor thinks there is some hidden in the castle.
BFG: The cannon is so big it takes dozens of men to put the ball in the barrel.
Cats Are Mean: Averted, Szaffi the cat is really nice.
Coin-on-a-String Trick: The guard is a recurring victim of this trope used as bribery, though it's done without the string.
Don't Tell Mama: Puzzola is often worried what his mother will say if she learns about his crimes. For a good reason: she's a torturer in Bologna's dungeons.
Dreadful Musician: The Baron's daughter drives the flute player to tears.
Dragon with an Agenda: When the Governor is about to shoot Jónás and Szaffi, Puzzola reveals he's a Not-So-Harmless Villain: he knocks out the Governor by opening his high pressure valve, takes his gun and attemts to claim the treasure for himself.
Expy: The protagonist, Jónás looks very similar to the titular character of Ludas Matyi, an earlier animated film of Attila Dargay. In Hungarian, they share the same voice actor too.
Baron Loncsár, like the antagonist in Ludas Matyi, is also a Fat Bastard aristocrat, and the two characters also share a voice actor.
Fantastic Light Source: When it's getting dark, Szaffi's foster mother summons some fireflies.
Harmless Villain: The Governer's henchman Puzzola has multiple murder attempts fail.
Not-So-Harmless Villain: While clumsy, Puzzola is a lot smarter and probably a lot more mean than the Governor, as he is the one suggesting him to kill the boy. He is admittedly an "intellectual criminal" rather than a murderer. And in the end he turns against his master to claim the treasure for himself.
Of Corsets Sexy: The Baron's daughter. Played for laughs, since she's rather unattractive otherwise.
Please Shoot the Messenger: In a variant, after Jónás fails to ask out the Baron's daughter, the father sends a letter via messenger pigeon to the Governor that he should kill Jónás. Szaffi the cat, however, attacks the pigeon and takes it to Jónás, who decides to deliver the letter himself, turning it into this situation.
Scenery Censor: Szaffi bathing again, it's a long scene.
Take That, Critics!: In an very anachronistic gag, Puzzola mentions film reviews among his preferred crimes.
Roma: The protagonist's surrogate family. Possibly the old lady in the swamp too.
We Named the Monkey : A plot point, since the protagonist thinks the black cat is Szaffi because they share the same name. Subverted in that Szaffi, the girl is named after her baby sneeze, but it happens to be the same as the name of the old lady's cat.
Why Don't You Just Shoot Him?: Puzzola is rather upset when he learns the Governor gave the castle to the protagonist instead of killing him.
Worthless Treasure Twist: Averted. While the protagonists learn a lesson about that the real treasure is love, they get the Sultan's treasure anyway.
Wrong Girl First: Jónás initially wants to marry the Baron's daughter Arzéna, until he realizes his heart really belongs to Szaffi.