(written from a Production point of view)
Paramount Stage 18 is located on the Paramount Pictures lot in Hollywood, California. It's noted for a number of famous productions, including Billy Wilder's 1950 film, Sunset Blvd. (where it was seen as Cecil B. DeMille's soundstage), the classic western Shane (1953), and Alfred Hitchcock's 1954 Rear Window. 
According to James Mees, the space underneath Stage 18 was used as a warehouse for Trek props and set pieces following their move from the warehouse in Burbank. The space was 12.000 square feet. ("Inside Starfleet Archives Year Six - Sets & Props", TNG Season 6 DVD special feature)
For the new production, Star Trek: Deep Space Nine, more space at Paramount Studios was allotted to the Star Trek franchise.
Stage 18 was first used in the pilot episode of DS9 for the various temporary sets needed for that production, including the main bridge of the USS Saratoga and Cardassian ship, and the Bajoran monastery.
The Temple of Masaka from the Star Trek: The Next Generation seventh season episode "Masks" was built on Stage 18. The set was later put to good use as the Albino's fortress in DS9: "Blood Oath". TNG's own Stage 16 was not free, as the village from "Thine Own Self" was built on it. (Star Trek: The Next Generation Companion)
In later years, DS9's version of "Planet Hell" - an interior planet cave set - was constructed on Stage 18, as well sets representing the USS Defiant bridge, quarters, sickbay, mess hall, and engine room, though the latter were torn down to make room for a Klingon starship interior and generic Cardassian setting. The stage also hosted the sets for Kasidy Yates' freighter Xhosa and a Bajoran temple. 
Three large sets were built within the stage and were central to most locations aboard the Enterprise NX-01: the bridge, engineering, and launch bay interiors. Paramount Stage 8 housed the remaining NX-01 interiors used in Enterprise.
The sets were constructed side-by-side, with engineering in the center; that set also included a small network of corridors, the transporting device, and decon chamber. The corridors that lined engineering were referred to in blueprints as the "Engineering Corridor".
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- Judith & Garfield Reeves-Stevens, The Making of Star Trek: Deep Space Nine
- Terry J. Erdmann, The Secrets of Star Trek: Insurrection
- Michael Okuda, A Brief History of Paramount Stages 8 & 9, StarTrek.com