Each bonus appears to be cumulative. I don't have a game far enough along to see what actually happens when I build a citadel or great bastion, but it looks like benefits diminish as defenses get more elaborate. Here's the Colopedia entry: "The first structure is the Chevaux De Frise that offers a 30% bonus. It is followed by a Stockade (60%), a Fort (90%) and a Fortress (120%). If adding a Citadel the bonus will be 150%, and adding a Great Bastion will raise the bonus up to 160%." The citadel has a 30% bonus on the fortress and the great bastion has a 40% bonus on the fortress. That's close to what you're saying, but not exactly. I'm guessing the bastion just gives a 10% boost over the citadel, but I don't want to change the Colopedia til I check the gameplay. The game keeps changing, which is good, but it means the Colopedia always has to catch up.