TOKYO - The deadly earthquakes that struck the southern Japanese island of Kyushu on Thursday and Saturday breached the walls of Kumamoto Castle which had previously withstood bombardment and fire in its four centuries of existence.The fortification in the city of Kumamoto has stood as one of Kyushu's icons ever since it was built in 1607 by Kiyomasa Kato. He was a veteran military campaigner and feudal lord who took part in the reunification of Japan, which had been ravaged by a century of war.Television footage on Saturday showed a large section of the stone wall housing the castle collapsed in a dusty heap.A powerful 7.3 magnitude earthquake struck southern Japan early on Saturday, killing at least 20 people, injuring more than 1,000 and trapping people in collapsed buildings, barely a day after Thursday's quake killed nine people in the same region.While the castle keep, which has so far withstood the series of quakes, is a concrete reconstruction built in 1960, many of the stone walls are originals from the 17th century. Sections of the outer stone walls had already been damaged by Thursday's earthquake.Long after advancements in firearms made such fortifications obsolete, the castle withstood artillery fire when it came under siege from a rebel samurai army during the Satsuma Rebellion of 1877. Much of the castle structure later burned down during the conflict, although the walls managed to stand firm.Now a popular tourist attraction, the castle's administrative office has closed the structure to the public. "Please do not go close to the stone walls as aftershocks continue," the office said on its Facebook page.