The soldiers arrived at the foot of the mountain before dawn.

Literal

Soldiers [topic-は] dawn-before [time-に] mountain [possessive-の] foot [destination-に] arrived.

Two distinct uses of に — one marking time (夜明け前に) and one marking destination (麓に). 麓 (ふもと) specifically refers to the base of a mountain or hill, a word worth knowing for geographical descriptions.