When the map generator created the rectangular rooms, it created a list of all these rooms. We now use that list to find a suitable room for the generators. The criteria is quite obvious: The room has to be at least 3x3 cells.If the map generator cannot find any suitable room, it will place a single generator in a random dead end. This rarely happens, but it can.
At this point, a request to build an empty base stops here. Empty maps are not used in Captive, but still supported by the map generator. This option was probably used by the programmer to quickly test the map generation layout. If a full base is requested, the map generator continues with the next steps.
