As anyone with even a scant knowledge of Japanese history is probably aware, the Battle of Sekigahara in 1600 was the "Big One." The absolute victory of Tokugawa Ieyasu made shoguns of him and his successors, who kept their hands firmly clutching the reins of power until they were wrenched away in 1868's Meiji Restoration.
After the conflict, Tokugawa was well disposed to those who had assisted in his ascent; less so to those who hadn't. And thus his trusted ally Ii Naomasa found his reward in the form of a hefty stipend plus Omi Province, centered on Hikone in today's Shiga Prefecture — just down the road from Sekigahara in present-day Gifu Prefecture.
Naomasa promptly did what any self-respecting/self-admiring feudal lord would do under the circumstances: he started building himself a bloody big castle. However, a stray bullet he stopped at Sekigahara led to his demise just a couple of years later, and it was left to his sons to complete the edifice that is today's biggest tourist lure in Hikone.