In theory, tunnels are a point A and a point B.They don't always appear as such, but they are more common in sci-fi or magic RTS games.

Good point.  5-6 teams may be useful, if using different ai levels though

create feature then people adapt... 

So, isn't the advantage of cavalry their high health? They cost extra due to their superior speed, but they also have a massive amount of extra health.

Is it? I can remember proper tunnels one could make only in Earth 2150, and an underground level in Heroes of Might and Magic… but then barely anything else, the Zerg tunnels in StarCraft, which I don’t think 0 A.D. should aim for (to avoid a teleporting effect).