Coastal is from coast plus -al. Piecewise doublet of costal. The adjectival suffix attaches the word to coast semantically, giving it the sense of being of, relating to, or characterized by the base. In use as an adjective, coastal denotes located on or near the coast, or relating to the coast. Typical contexts include “They live in a small coastal town”. The meaning stays close to the form: it applies the etymological material to located on or near the coast, or relating to the coast, keeping the visible parts legible in ordinary use.