The idea is to be able to build siege weapons and take down the enemies fortress if you are a good. If you play less good and you lose lots of units you get iron to buy special units to have a good counter. So basically it's a really well balanced scenario and can take a while to win. ^^
The Scen in it's current version is fully functional. So if anyone want's to try it, just leave a msg.
I also have a coop tower def but I go to remake it because it appeared out of a "just for fun" creation and fits the crazy game style for me and my friend.
I am doing a new one soon as well. A coop def scenario where both player have to keep a fortress together vs. incoming (endless) waves of enemies. Units can be purchased with gold and are way stronger than the enemy. (Like special soldiers). Therefore they are expansive. The enemies units will inc. in life, damage and armor over time. To hold against it we will be able to upgrade Longsword, pike, longbow, crossbow, cav and knight with damage, HP, armor and range for certain amount of gold of course. (Like I said it will be endless. So the scale of the enemies HP will goes way higher than the upgrades you can purchase)