demand surrender

[Belgae - the furious ones]

i besieged a city with napoleon, demanded surrender, then the garrisoned army left the city but was still within range of my armies, meaning that i could attack them now, in the same turn, but this time on the open field. I saved and attacked just to see whether there were any penalties in doing such a thing. i did not see any.

is this not strange? they have the advantage of defending in a fort, but being heavily outnumbered, they are given the chance to leave unharmed if they just give up the fort. they give up the fort, and immediately thereafter they are mercilessly massacred, with no apparent downsides to this course of action. I think this is a nice feature, but it just seems so silly when it means you can just eliminate them afterwards in the open field.

or am i missing something?

[Pathfinder]

You're not missing anything. Surrender is a good mechanic, but the way armies retreat is not - they don't go nearly far enough. Of course if you exhausted your movement getting to the city the garrison would escape.