From the castle, I could see the entire river curving gently as it flowed along the foot of the mountain.
Literal
Castle [from-から], its foot [path-を] gently curve flow river whole [subject-が] was-visible.
見える 'to be visible, can be seen' is an intransitive/potential verb taking が, not を. The long relative clause [そのふもとをゆるやかに湾曲して流れる] modifies 川 'river,' containing two verbs linked by te-form (湾曲して流れる 'curves and then flows'). を on ふもと is unusual for a place — it marks a path of motion, as in 道を歩く 'walk along the road,' meaning the river traces its course along the mountain's foot. ゆるやかに 'gently' modifies 湾曲する 'to curve.'