FYI battle maps are all saved in the game, there is a fixed range. They're not big files, actually, seem to be just a collection of "texture references" which call up the graphics from the textures folder, and depending on the climate type, different tiles will be called - so the map can be the "same" map yet still have temeprate/lush/arid variants from the same map file.

At the start of battle, the game will pull out a random map from the group which is appropriate to the battle (province and border info) - ie terrain type, climate, whether it has a river or not etc. The map groups contain mostly single-bridge maps, but there are a few double-bridge ones, so it will sometimes randomly select one bridge, other times it will randomly select two bridges. Any apparent "development" over time is purely coincidental.

For a bit of real fun in a two-bridge battle, line up your artillery between the two bridges, then send a fast cav unit running between the two bridges