Amalickiah’s strategy was not to force a strike up the Sidon valley, but to control this lower access point. He concentrates on the seashore cities so that he could operate from a secure base without worry of Nephites in his rear. Ludlow notes the curious comment that Amaliciah prohibited the Lamanites from moving against Nephihah, but it appears in the list of conquered cities:
“Although the historian of this part of the record mentions in Alma 51:25 that the Lamanites decided not “to go against the city of Nephihah to battle,” in the next verse the historian records that the Lamanites went on “taking possession of many cities, the city of Nephihah, and the city of Lehi … ,” etc. Although it is not clear whether or not the historian intended to include the city of Nephihah in this list, it is evident that the city is captured later by the Lamanites. (See Alma 59:7-9.)” (Daniel H. Ludlow, A Companion to Your Study of the Book of Mormon [Salt Lake City: Deseret Book Co., 1976], 236.)
The most likely scenario is that the coastal towns/cities were taken first, and with those bases secured, then Amalickiah was ready to move up the Sidon valley.