By 1474, cannons had been introduced in Mdina. Other extensive preparations for an attack were made in the 1480s, when the fortifications were once again improved under the direction of Sicilian military engineers. At this point, some buildings in Rabat were demolished to clear the fortifications' line of fire. By 1522, the fortifications were being modernized with the construction of embrasures. However, the walls were still regarded as obsolete, since they lacked bastions and could not resist bombardment from modern artillery. Most of the medieval walls of Mdina were gradually dismantled between the 1530s and 1720s, when the city's fortifications were being upgraded by the Hospitallers. One of the most significant visible remnants of the medieval fortifications is the Greeks Gate and the surrounding curtain walls, which still retains its medieval form apart from the outer portal which was built in the 18th century. Parts of the double walls, including the remains of two gun loops, still stand between the gate and the Torre dello Standardo. Most of the northern and eastern walls date back to the medieval period, although some sections were rebuilt by the Hospitallers. The northern ramparts contain a medieval wall tower, while the remains of Byzantine antemurals and the foundations of a late 15th-century artillery platform have been found in the eastern walls.
Question: How many decades did it take to gradually dismantle the medieval walls of Mdina?
Answer: 19