The catwalk was the unofficial designation for the lateral access conduit running the entire length of the warp nacelles of an NX-class starship. Located at the center of the nacelle, just below the ship's warp coils, the catwalk was accessible through a single vertical Jefferies tube.
Protected by an outer shell composed of an Osmium alloy, 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. (ENT: "The Catwalk")
In 2152, the crew of the starship Enterprise took refuge from a deadly neutronic storm within the catwalk. Utilizing a makeshift command center prepared by the ship's Chief Engineer Commander Charles Tucker III, Captain Jonathan Archer, First Officer T'Pol and Helmsman Travis Mayweather were able to control Enterprise's navigation through the storm. (ENT: "The Catwalk")
Later in 2152, Archer once again led his crew into the catwalk when Enterprise was invaded by non-corporeal beings. 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 fan webpage Ex Astris Scientia, scenic artist Doug Drexler states that his original intent for the design of Enterprise would have had the ship's nacelles, pylons and double hulls uninhabitable. He also states that the sections would have been accessible only in environmental suits from the outside of the ship through the engineering EV staging area located at the extreme aft portion of the saucer section.