I thought that it would add a cool strategic layer to ground combat if there was an unique defending strategy card directly tied to an unique military improvement. 


You could build an improvement and it would give you an additional defensive strategy in addition to Conscription, Tactical Surrender and Protect System.


Examples for improvements:


Mist generator: -35% troop loss on defending empire, -20% damage on defending empire

Electronic scramblers: +30% damage againts invading tanks and planes

Biological weapons: +30% damage on invading infantry


You could place these improvements on chokepoints or other significant hotspots on the map but only once (for balancing reasons). And it should be a big investment to avoid spamming all the improvements in the same system.