Multi-layer / Multi-level Pathfinding. Pathfinding: Grid-based (CCmpPathfinder) with passability classes tied to terrain. Bridges must support land units on top while blocking naval units below. Dynamic changes: Construction, destruction, or damage must update passability in real time.   Performance: Avoid full global recomputation on every change. It would be a huge problem to be constantly destroying and building bridges.   Other ideas with bridges and walls. Portals / Vertical Links: Define connection points (ramps, stairs, ladder zones) between layers. When a bridge is built: Mark upper tiles as walkable for land units. Keep lower area impassable for ships. Destruction: Remove the upper layer passability and force path invalidation/recalculation. This is similar to how Stronghold, Age of Empires IV, or modern Total War titles handle elevated walkable structures.   ii- Dynamic Navmesh + Grid Hybrid Use a grid for fast coarse terrain movement. Use a recast/detour-style navmesh (or equivalent) for precise dynamic obstacles and walkable surfaces. On bridge build/destroy: Locally re-bake or update the affected navmesh region. Add/remove off-mesh links for vertical movement. Advantages: Handles complex shapes well; good support for dynamic environments. Many modern engines and assets (e.g., Unity's A* Pathfinding Project or Recast) support this. Another interesting alternative is: Passability masks / cost maps + obstruction system. It sounds complex...

Stronghold , Praetorians anda AoE IV They have good concepts of how to make walkable bridges and walls.

I'm not saying there aren't good ideas in mods, but they never make it into the base game.   That's why I was going to sit down and draw and make sketches, because apart from giving opinions and designing mechanical concepts, I can express myself by drawing doodles.

You should check out Classical Warfare   I think it might be the main reason people are "turned off" by it, reluctent to try it.  Its not as simple, almost identical civs, play styles as you find in the current base game.  It still has a lot of polishing to do of course.    

That's ok