Matsumoto Castle in Nagano Prefecture is more vulnerable to an earthquake than previously thought and requires work to strengthen its resistance, officials of the city where the national treasure is located said Thursday.

A recent earthquake resistance test has found that the castle's main tower could fall in the event of a quake measuring upper 6 or stronger on the Japanese seismic scale to 7, according to the officials of the Matsumoto Board of Education.

The test results showed the resistance levels at some parts of the main tower, or tenshu, are insufficient if the castle is hit by a quake of this scale — nearly as powerful as the one that triggered catastrophic tsunami in northeastern Japan on March 11, 2011.

The city will begin working shortly on enhancing quake-resistance structures of the castle and improving evacuation plans involving the major sightseeing spot.

Matsumoto Castle, which is believed to have been built more than 400 years ago, drew about 990,000 visitors in fiscal 2016 through last March. The last large-scale repair and maintenance work on the castle was conducted from 1950 to 1955.