This is a secret level accessible from Occupied Territory.
There is a door in each of the four outer regions of the level (Alpha, Beta, Gamma, and Delta). Behind each door is a switch, as shown in the image. There are four of these outer region switches to activate, and each one will open a door in the inner region of the circular level. Behind each of the doors in the inner region is another switch. The second set of four switches must all be activated in order to open the central area containing the nuke button. So, in total, there are eight switches that must be activated to finish the level. The switch placement is symmetrical, so they are all easy to find.