Complex sentence: However, in the past couple of years, the castle defenses had been increased by Suganuma Sadamichi, and the castle held out for several weeks, only reaching the point of surrender after Takeda sappers tunneled into the moats, draining them of water.
Split into simpler sentences: However, in the past couple of years, the castle defenses had been increased by Suganuma Sadamichi, and the castle held out for several weeks. They only reached the point of surrender after Takeda's sappers tunneled into the moats, draining them of water.