Building a city on a hill?

Why does the AI(atleast im not sure if i could) get the +50% defensive bonus when it builds a city on a hill? I know it kind of makes sense but especially in the early game the +50% is really important. And you don´t get the defensive bonus if you build a city to a forest.

a city on a hill is harder to assault. A forest would have to be removed to make room for a city so such a city is not harder to assault. There is the reasoning behind the mechanics.

Is there both attack and defensive bonuses for hills? IIRC from a save yesterday, the troops from a city on a hill just got attack bonuses, and not when attacked.

There's a defensive bonus too.