RR: 8.0
Now, this is the first original map to be made available for Nerf Arena since the game was released, instead of being an adaptation (or "remix") of an Unreal Tournament map, so one can forgive it the occasional lack of smooth construction, innovation, or general steadiness.  However, although the current release is only a "beta" (so a more polished final version may come along), there are no such real flaws to forgive.  Instead you have a large and atmospheric building to explore on two levels, with tricks and devices never before seen in Nerf.  Rooms that are fatal if somebody on the upper floor touches a particular control panel, for example, and moving floorways.  There are bonus areas that you can only reach if a friend blasts you up there with a ts; walk along the edge of the top wall and view the harsh alien landscape outside, or jump across onto the rather fine statue that decorates the largest open area.  There are all the weapons you could wish, plus lifts (elevators, okay?), teleportation, neat hiding places, bouncing pads, points-targets, and masses of atmosphere, making this an arena you can't afford not to have!  Full marks to LeStat (now better known in UT mapmaking circles as "HillGiant") for an atmospheric, spooky treat.
          Due to the lack of a hyphen, this map comes up on the server list as PMOrbitspire rather than just Orbitspire - not a great distance apart alphabetically, luckily!  And the other point I should mention is that this map recently (early 2001, as I add this) won a "Kudos" award from SLAM as the best new map for NAB, so if you don't have it, get it!