I can't reproduce this. Air units ignore building defense. So when an air unit is selected, CvCity::getDefenseModifier computes the modifier without buildings. If that's 0, then no modifier is displayed on the main map. However, as a city takes damage, the defense modifiers with buildings and without should gradually converge; so if it's 0 when an airplane is selected, it should also be 0, or at least very close to 0, when a Catapult is selected. How so? chg is negative and gets added to the defense modifier. So min(0,... should be correct. For collateral damage Siege units, I've changed the floor function to round (to nearest) a while ago and I forgot to do the same for Air units. My goal is that e.g. a Fighter attack (BombRate 8) on a city with a 60% non-building defense modifier result in a 52% non-building defense modifier. Rounding to nearest seems to accomplish that, whereas rounding down (floor) or up is often off by 1. But that's a small issue and probably not the bug you're looking for. When the AI contacts you at the start of a turn? How slow, and which part? The offer/ demand or when negotiating at the trade table? Or the Foreign Advisor screen? (Which tab?) -- In a quick test, I did see it when hovering over my own name on the scoreboard or portrait (screenshot attached) on the Foreign Advisor. Looking at the code, I also don't see why it wouldn't work. It would be easy to tell with a savegame. Good catch. I had removed some redundant code that the Advanced Combat Odds (ACO) mod and BtS had in common (or so I thought). ACO shows negative modifiers because that's how the calculation actually works; however, ACO shows those negative modifiers under the defender – showing them under the attacker (Rifleman) is nonsense. Edit: Thanks. For a new player you already seem to be quite well familiar with the UI and its little nooks and crannies.