"Mathias Sandorf" by Jules Verne is an adventure novel published in 1885. When petty criminals intercept a secret message, they expose a Hungarian conspiracy against Habsburg-Austrian rule, leading to arrest and execution. Count Mathias Sandorf escapes and disappears into the Adriatic. Fifteen years later, he resurfaces as the wealthy and mysterious Dr. Antekirtt, commanding an island fortress with advanced weaponry. With two French carnival performers as allies, he navigates the Mediterranean seeking vengeance against those who betrayed his cause and destroyed his friends.
Wikipedia page about this book: en.wikipedia.org/wiki/Mathias_Sandorf