When Mehmed II ascended the throne again in 1451 he devoted himself to strengthening the Ottoman navy and made preparations for an attack on Constantinople. In the narrow Bosphorus Straits, the fortress Anadoluhisarı had been built by his great-grandfather Bayezid I on the Asian side; Mehmed erected an even stronger fortress called Rumelihisarı on the European side, and thus gained complete control of the strait. Having completed his fortresses, Mehmed proceeded to levy a toll on ships passing within reach of their cannon. A Venetian vessel ignoring signals to stop was sunk with a single shot and all the surviving sailors beheaded, except for the captain, who was impaled and mounted as a human scarecrow as a warning to further sailors on the strait.