There are several synonyms for the word "builtup" which can be used to describe a densely populated urban area. Some common alternatives include "urbanized," "metropolitan," "developed," "cityscape," "conurbation," "cosmopolitan," "industrialized," "builtin," and "municipal." Each of these words conveys the idea of a crowded and developed urban environment, with buildings, infrastructure, and busy streets defining the landscape. Whether you prefer the more formal "metropolitan" or the trendy "cosmopolitan" to describe the atmosphere of a bustling city, there are plenty of options available to accurately convey the sense of a builtup area.