Generally, in some indoor stadiums or arenas or theaters, if you will...there is a walkable space in the roof or the area directly above the stage...from which many technical activities are performed...
Above the stage is the Rope Loft or Fly Loft. To day it's usually referred to as The Grid. The working/walking areas are usually referred to as The Bridge. If you say "Up on the grid" riggers and techs will know what you're talking about.
Also see: http://en.wikipedia.org/wiki/Fly_system
I would refer to such a platform as a "gantry".
A bridge-like overhead structure with a platform supporting equipment such as a crane, signals, or cameras:
"the TV cameras on gantries alongside the 17th and 18th holes"
Castles and cathedrals have “allures/alurs/alures” aka “wall-walks” around their perimeters near/at/above roof-level to facilitate foot-traffic and movement of weapons/equipment. While they wouldn’t apply to theaters/stages and are perhaps not designed for modern technical activities, stadiums and arenas probably have similar elevated walkable ways around their perimeters.