Geographic: Zarahemla lies on the West bank of the Sidon. Since the hill Amnihu is east of the Sidon, it is a hill guarding the approach to Zarahemla from the East.
Using Sorenson's geographic correlation, the Sidon flows down a valley bounded by a mountain range on the east. Thus the guarded approach is not from the East directly, but rather from the North on the East side of the Sidon.
As a raised hill in the valley floor, the hill Amnihu would provide elevation for vision of approaching troops, as well as the higher ground guarding that approach to Zarahemla. (see Sorenson, John L. An Ancient American Setting for the Book of Mormon. 1985, p. 154 map 7).
Military: While it is not stated, we may presume that the Nephites would have used fairly standard military practice and have occupied the hill Amnihu.
The Amlicites were the aggressors, and this means that they would have traveled to Zarahemla to engage the Nephites. The Nephites would be defending territory, and this close to Zarahemla should have created a defensive position on the hill. It is quite probable that this hill was near the main way into Zarahemla from the East.