The "1 unit per hex" thing would work if a hex actually represented space that 1 regiment (say, division) occupies. But that's not the case, a megapolis irl can hold several hundred divisions on its territory. And if you think of 1 unit as an army group, then you should be able to combine all kinds of soldier types within a single unit - have you seen a 100.000 army consisting only of archers? So if a city occupied 1 hex when just founded and 100 hexes (10x10) when developed into a big city, and if it could work a radius of 50 hexes in each direction, and cities would have to be placed 80-100 hexes from each other, then the 1 unit per hex system would work. But that would be a tactical game, while civilization is and has always been a strategical game. Also if we want to stick to realism, battles in ancient times and in modern times are different. Battles in ancient times were fought just like in civ 4 - two countries build a stack of units (two 100.000 armies), those armies meet on a field of battle (a single tile) and one army wins, the other loses. The war doesn't cover a huge territory, battle is fought on small clusters oа land. Modern warfare, on the contrary, is about front lines stretched across whole continents, which is what they tried to achieve in civ V. But still those front lines do not work with 1 unit per hex system. They need to put a limitation of 3-5 units per tile/hex, as in reality a single part of front is always protected by several types of troops - say, infantry, some tanks, some anti-tank inf, some paratroopers, etc. No general would operate a front line where first 100 km are defended by infantry, next 100 km by cavalry etc.