The coastal town has become very built-up over the past decade with tall buildings everywhere.

built-up 是由动词 build（建造）的过去分词 built 加上副词 up 组成的形容词。build 表示"建造"，up 表示"向上、增加"，built-up 合起来表示"建筑物密集的、高楼林立的"，形容一个地区经过大量建设后建筑物密集的状态。