Thanks to Melf_Himself for bringing my attention to Iron Grip Warlord an independent game from Isotyx. The first map can be donloaded and played for free including online play while the full game costs $24.95. Well worth the price in my opinion.
Iron Grip Warlord is a curious hybrid of rts, fps and tower defence game set in a steam punk universe. It has standard rts elemetnt of resource gathering (by killing stuff) and structure building, you also play in FPS mode yourself as one of the troops. The tower defence bit comes from the fact that you must defend your base against an endless succession of enemy troops and armour who thread their way through your city to annihilate your base. In practise it plays like Company of Heroes on acid and is a total blast as you run around building, repairing and shooting all the while trying to plug gaps in your defences.
Best thing of all is that it is designed for multiplayer co-op mode which is tremendous fun. I have only played disorganised "pick up games" where it is pretty much every man for himself but I imagine that the game would lend itself very well to organised team play.
In single player mode your team gets padded out with bots but they are pretty dumb leaving you to do everything yourself so it is much harder than multiplayer. Being a masochist I have spent several hours trying unsuccessfully to win a round on medium difficulty settting. I haven't got there yet but I am getting closer! My current strategy relies on careful placement of land mines and judicious use of the bazooka for killing enemy armour before it gets to the top of the hill.
Oh by the way it took me ages to figure out how to get extra ammo. If you are in the same dilemma then you need to build a support station. These are great as they replenish health and ammo. I reccommend making one in a safe spot first thing and running back regularly to restock.