Multiple collision areas within plane

Hi there,

In my retro-styled "pseudo 3D" game, I have a flat plane as the playfield with the walls/doors etc drawn onto it, like the old SNES Mode7 days. The question I have is, what is the best way to do collision? For example, certain areas (like water etc) slow the main character down and others make him stop. I have built the walls with invisible objects and was going to do the same for the water areas etc, but the problem is, they are a lot more detailed... not just cubes... this would take MANY gameobjects to achieve correct collision detection.

Is this the way I need to do it or is there some way to detect colours like in the "olden days" and use that instead? Perhaps a collision map as opposed to a light/bump map? Is this achievable or is collision only determined by meshes?

Cheers!!!