Terrain defense bonus?

[Grotius]

The manual mentions a terrain defense bonus, but I don't see specifics on what terrain grants what bonus. It says simply: "Defense Bonuses are provided by the terrain and resources a defending
unit is occupying, with forests, mountains, cities and fortresses providing the most."
But are there more specific guidelines? E.g., does a mountain double defense strength? Triple it?
What effect do jungles have?

The combat UI in-game provides some more clues, like the little river symbol indicating you're attacking a river. But some more precise numbers would be helpful. If nothing else, what about jungles?

[Sugar]

Rightclick on the hex - properties.

[Grotius]

OMG, I never knew you could do that. Thanks!