What provoked the question on my part was that in my last game I had a lot of catapults that I had never used or promoted, built in the appropriate age for defensive purposes. Much later in the game, I was the dominant military power. I had dozens of mobile artillery, modern armor, air power, mobile infantry, a huge navy, etc. While the number two military was mostly infantry and a few tanks. It was way too expensive to promote those catapults at that point. I was wondering at the time, if I should keep them to throw at an invader's SOD before hitting it with the mobile artillery or just disband them. It is just a theoretical question, since I was playing for a peaceful victory and only intended to use the army if I was attacked, which did not happen.