Actually, thats wrong. Because...

Possible, that'd be something to have an AI look into if you want this implemented. Also note that for a bridge, pathfinding isn't really needed, so just to have the units "walk" across the bridge should be easy. Not really. That's a bit difficult, I've done it before tho: A: make the "bridge" a unit B: give it Build Restrictions like a regular building C: give it a unique "amphibious" passability class in passability.js D: work out any small details like cost and foundation and stuff.

This would look goofy as hell :P. I think (hopefully) the engine can be modified to deal with these things properly, considering there's the Immortal air-walking exploit around right now, units walking on top of buildings shouldn't be impossible to implement, unless I'm missing something. In any case, and as I've mentioned before: prohibiting the positioning of pillars in deep water should be enough constraint (this allows a very short bridge over deep water). Going a bit chronologically, rafts pulled by a rope would have come quite early (a much later version of this was apparently used by Hannibal to ferry his elephants across the Rhône). Wooden walkways in shallow water were made from at least the Neolithic. In the Bronze Age, stone bridges would be made to deal with uneven terrain (Arkadiko Bridge). Workers should be able to build all these things. To build pillars (from wood, later stone) on medium water, fishing boats would be too small and unrelated for the task, if anything I'd prefer the use of merchantmen, which were (obviously) cargo oriented, and were historically used for a variety of things (even warfare when needed). They could also be used for earlier Pontoon bridges. If a bridge was meant to be permanent, there were ways to allow navigation: either movable (pontoon) parts, or maybe simple enough for the game, enough clearance, since masts were removable and most ships were at most around 3 m above water, not counting later huge cargo ships, but soon enough the Romans would have mastered great arches.

But maybe we could start something, we would just have to figure out how to allow building certain structures underwater. Also, is there any structure that units can walk on top of? I can't think of any

@Asher darn! You are right! Walls can have gates, so why not have a gate under water for a ship to undercross the bridge? The animation could be challenging though...