Built on the predecessor Fukashi Castle and substantially expanded by Ishikawa Kazumasa and his son Yasunaga in the 1590s, the present keep was completed at the end of that decade. Several Edo-era clans (Mizuno, Matsudaira and others) held it, but the castle survived intact. After narrowly escaping demolition in the Meiji era — locals bought it back — it was designated National Treasure in 1936 and again in 1952. Japan's oldest surviving five-story keep, and one of the five "National Treasure castles."
The inner bailey is wrapped by twin water moats; the keep rises five tiers and six floors in black lacquered wooden cladding. A network of spring-fed channels extends from the moats into the town, serving drinking water, sake brewing, dyeing, fire prevention, and defense from one hydrology. The nickname "Karasujo" (Crow Castle) for the black exterior reads as aesthetic — but it's also rational: camouflage against winter snow and summer shadow.
Matsumoto's defensive philosophy is "maximize the water." Without a hill or mountain to lean on, the moats stop attackers through depth and volume alone. The keep is less the final redoubt than the symbol of a town-wide water network — every feature of the city does multiple jobs from one resource (water).
Matsumoto's water grid is the ideal model of "infrastructure that does multiple jobs." What modern smart-city designs try to achieve in software, Matsumoto achieved in physical layers six centuries ago. "One layer working four times" translates directly into modern SRE and platform thinking.
From Kuromon gate through the inner garden to the keep entrance, about 10 minutes. Inside the keep, the stairs become ladders — shoes off, carried up. The top floor faces the Northern Alps head-on and confirms why the castle "chose water over a mountain." Winter dawn — the frozen moat reflecting the keep — is the picture.