The biggest pacing problem of the game. Defense Stat.
The biggest pacing problem in this game is how the defense stat works. My understanding of how it works is that it takes 1 damage away from whatever defense type it is. The problem with this is that it rapidly renders units completely useless, sometimes with a single tech level. You can see this very clearly with City Archers. They go from dealing 14 damage when there's no armor involved to 1 plink a shot.
It also becomes very apparent as sovereigns go from having little defense to a large number, possibly with stoneskin stacked on top. Making them essentially invulnerable to militia and the like. There's little point in defending your cities when the units that are used to defend them cannot do damage anyways.
I think defense needs to reduce but not eliminate damage. That way, weaker units do not become useless on the battlefield.