mountains:

this is where your troops go to hide from the wrath of my armies. You think you have a win, but it's a standoff. I don't have to attack you. I can just go around a different way, take your cities, and eliminate your faction.

Unless you're defending mountain passes from the Mongols, mountains don't come up very much. I only use them when defending against the suicidal ai, rare cases that I am on the defensive.

So the situation is rare, and a stalemate, not a loss.

Next: Forests.

Bwahaha! I could easily win. The only things I fear are archers and heavy cavalry, neither are effective in forests.

But neither are light cavalry, right?

That's debateable. I can outrun, outflank, and generally outmaneuver infantry even in dense forests. And once they move, they expose their position. There will be no ambush, as I will send in a scouting unit who can quickly get away. Once your position is exposed, I fear no ambush. Then it's just a matter of quickly striking infantry with lance charges and running away until their general exposes himself, which then exposes him to flanking maneuvers. With the general dead, hit and run tactics will win the day. Eventually the infantry will get tired of chasing the cavalry.

The only solution I see that works with unmaneuverable infantry is the Giant Schiltrom formation, with the general in the middle, on a hill in the forest. With no exposed flanks, I cannot win. But then again, I don't have to charge to my death in this case either, and we have the same situation we had on the mountain. You have a defensive advantage, and I don't have to fall for it.

You cannot force me to attack you unless you're besieging me, so these particular exceptions are moot.