Protected by an outer shell composed of an osmiumalloy, the catwalk was used as a maintenance shaft, though it was only operative if the ship's warp drive was shut down. With warp systems online, the catwalk was subject to extreme high temperatures and was not suitable for habitation. The catwalk was divided into several compartments. (ENT: "The Catwalk")
Later in 2152, Archer once again led his crew into the catwalk when Enterprise was invaded by non-corporealbeings. Unable to penetrate the catwalk's protective deflector shielding, the beings were kept at bay long enough for Archer and his crew to repel their attack. (ENT: "The Crossing")
In "A Close Look at Enterprise NX-01" from the website Ex Astris Scientia, scenic artist Doug Drexler stated that his original intent for the design of Enterprise would have had the ship's nacelles, pylons and double hulls uninhabitable. He also stated that these sections would have been accessible only in environmental suits from the outside of the ship, through an engineering EV staging area located at the extreme aft portion of the saucer section. 
From the Star Trek Encyclopedia (4th ed., vol. 1, p. 125), "The catwalk was a large set constructed on Paramount's Stage 8 for the episode of the same name. It was left standing for several weeks until it was taken down after it was used for "The Crossing".