What type of building is a stoa?

A covered walkway or portico.

Explanation:

A stoa is a type of structure that was common in the architecture of Ancient Greece. This was a walkway that was covered in order for the public to use. It is similar to the idea of a portico. Stoas usually surrounded buildings in a protective way, and were places where people could stroll, sell their goods, display artwork or gather. They were particularly popular due to the fact that they were located around marketplaces or agora.

artists followed a set of rules to achieve ideal proportion.