Pop-rushing is like cash-rushing in the sense that it's "all or nothing": to complete the current build, you must whip as many citizens as needed (1 citizen = 20 shields). But unlike cash-rushing (where you can empty your entire Treasury if you choose to), it is not possible to whip more than half a town's population in one go (speculation: hardcoded to prevent the AI-Civs from killing themselves). This 'half-pop-point' restriction can be circumvented by the human, by using "short-whipping" over >1 step, first to a cheaper building/unit(s), then again to the final target. But this can only be done if there are convenient intermediate-builds left for that town — ideally with shield-costs in multiples of 20 — at the player's current tech-level (e.g. 20s = Spears, Archers, mil-Barracks, Explorers; 40s = non-Mil Barracks, Maces, LBMS, Cannon, Sci-Libs; 60s = Grans, non-Rel Temples, etc.). If not (e.g. Pyramids, ToA, and SunTzu are all under the player's control, and/or they haven't got Iron, or Invention/Astro, and/or have progressed to/beyond gunpowder-based weapons already), then the build must be whipped to completion in one step. If @CKS could whip a Col directly (120 shields = 1 shield + 6 citizens), there must therefore be at least 12 citizens in that town, and the whip-rush would leave her with 6 citizens. That would not be enough for a Lib (80s = 1s + 4 citizens), unless her Civ is Scientific (40s = 1s + 2 citizens). But even if she is Scientific, whipping the Lib to completion first (12 - 2 = 10 citizens) would still leave insufficient citizens for a direct Col-whip. Yes. Always The Persians never really got going on this map, though: squeezed between the Mongols, Iros and the Ottos, and without Iron, The Jerk never had much of a chance. That said, the last 2 towns I took from him had originally been Iroquois — Hiawatha also having been hamstrung by having no Horses (I got to them first!).