Mitsunari launched a large-scale repair project, constructing a towering castle with a five-storey (others claim it was a three-storey) central donjon on the mountain's summit, which lead to the saying 'Mitsunari outdid himself about two things, Sakon SHIMA and Sawayama-jo Castle.'
三成は大規模な改修工事を行い、山頂に五層（三層説あり）の天守が高くそびえたつほどの城を築き、「三成に過ぎたるもの二つあり、島左近と佐和山の城」と言わしめた。