Kendon
Prince
the last two levels of city forts, citadel and bastion, don't give the proper defense boost. They only give 30 and 40 percent.
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.