I just trawled through MapRaider looking for this map and the nearest thing I found was this - http://www.mapraider.com/maps/unreal-to ... NS-PM-Lore
... I think that shows the huge base defence gun that each base had in the map I'm thinking of. I've no idea why its not still in my map folder as everything else is.
The Onslaught map had two heavily built bases, big stone walls. Each had one of these huge cannons on it. The map was a brightly lit, medium sized, desert map with not that much in it apart from the Onslaught nodes.
So, anyone ?