The catwalk is the nickname both for a maintenance shaft inside a warp nacelle and for a maintenance walkway on Deck 35 just above the center of the warp core in Main engineering onboard Galaxy class starships.
In 2152 the crew of the Enterprise (NX-01) had to take refuge for eight days inside the catwalk when a deadly neutronic storm threatened the ship. (ENT: "The Catwalk")
In 2368 Lieutenant Commander Geordi La Forge was feeling dizzy and almost fell off the catwalk. He was rescued by Ensign Fletcher. (TNG: "Cause and Effect")