The soldiers arrived at the foot of the hill before dawn.
Literal
Soldiers [topic-は] dawn-before [time-に] that hill [possessive-の] foot [destination-に] arrived.
夜明け前に marks time ('before dawn') and 麓に marks destination — the basic two-uses-of-に pattern. その丘 ('that hill') uses the demonstrative for a specific hill known from context. ふもと is written in hiragana here rather than its kanji 麓 — both are common.