Cloud CItadel is the sixth zone, unlocking at 600 Starpower.
Units unlocked here
As a battleground
Cloud Citadel is a spacious, open battleground with a main chokepoint in the middle of the map, created by gaps on the top-right and bottom-left corners. There are no mountains on this map.
SENSO-Gate and Extractor locations
The SENSO-Gates on this map are on opposite corners of the map, with one of each players' Extractors near gaps.