What architectural feature consists of a series of columns surrounding a building?

A colonnade is a significant architectural feature characterized by a row of columns that support a roof or entablature. This configuration is often used to create a transitional space that connects different areas of a building or provides a walkable area along the façade. The aesthetic and structural function of a colonnade contributes to the overall harmony and stylistic language of the architecture, making it a prominent element in classical designs.

In contrast, an ambulatory refers specifically to a walkway, often around the apse of a church, and doesn't necessarily involve columns surrounding a building in the same way. An arcade is a series of arches supported by columns, often used to create covered walkways, but it differs from a colonnade by its arch-based design. A portico, while similar in that it also involves columns, typically refers to a porch-like structure that provides entry to a building, often featuring a roof supported by columns. While both porticos and colonnades utilize columns, a colonnade implies a broader and more extended series of columns that may not necessarily define an entry point. Thus, the defining characteristic of a colonnade as a continuous row of columns makes it the correct answer in this context.