So, archers CAN fire across hex boundaries, on their turn. As can City-class siege (presumably mounted on the walls).
The immediate point that comes to mind is attacking a City has certain 'rules'. That is- don't do what Ansom did- end turn right next to the walls. Ansom got away with it because GK had very limited archery support, but free hits off your turn from those archers and City siege would be painful and pointless.
Limited ammunition becomes critical. If you can huddle behind shields and decent armor, or I guess in Erfworld terms stack enough defensive bonuses, you can weather the storm of arrows by cycling wounded troops back out of range or behind 100% cover, then heal on your turn and cream the archers in the neighboring hex unless they flee like bunnies before your wrath gets there. So, overpowered? No. In fact, if your ammunition only repops at turn start like everything else, you might strongly consider NOT firing off turn to save your shots for a direct engagement, where the enemy can't pull the stunt I just outlined and actual losses can accrue to them.
That is, unless you have archers with high leadership bonuses and a dittomancer to double or more their shots. At which point firing off turn makes more sense

. Wanda is definitely in a world of hurt.
As an aside, if you, say, have a catapult and are out of ammo for the day, would any handy (I dont know, pick something random)
cow be a viable missile weapon? Parson commented on picking up a brick and tossing it. Can you scavenge 'ammo' in limited or creative ways from a hex?
I choose to believe that the limited exception of movement for Turnamancy comment leads credence to my theory that the King World spell only refreshed movement points, not a full turn refresh. Which would make King World significantly less powerful.
As to things freezing at the hex boundary, we know that notes can be passed hex to hex, off turn. So it may be that the hex boundary just sets the kinetic energy to zero, but then you could tug the arrow (or note) through to the other side if you felt like it. Careful doing that with things that have serious /potential/ energy. Like a big rock sitting on the 'roof' of a city zone above your archers. Or dragon corpse.
Which brings up how 'tall' the roof is. Unfortunately, it doesn't seem very tall at all- Jillian when talking to the King was pretty near to the floor without crossing the boundary. So laying corpses (battlecrap, bubblegum) on the city zone and then pushing it gently through might not work out pure awesome, but still has potential.
Hrm... pretty long, need to drink less coffee

I'll cut it off here.
Enthar