Complex sentence: Masamune was not happy with his previous stronghold, Iwadeyama, which was located in the northern portion of his territories and was difficult to access from Edo (modern-day Tokyo).
Split into simpler sentences: Masamune was not happy with his previous stronghold, Iwadeyama. Iwadeyama was located to the north of his territories and was also difficult to access from Edo (modern-day Tokyo).