There are some types of terrain that favoures the defender, like mountains, marshes, swamps, forest, winter and snow. They act like giant fortess and protect the defender. At the same time, they slow down the attacker. Not only do they attack uphill, but also a lorry with supply need ten times more fuel to drive in Mountains and snow compared to Plains.
Taking this to A&A will be like this:
Defending units hit at 4 or less in Mountains.
Attacking units hit at 1 in Mountains
All units move 1 Space only in Mountains, no blitz
I think only Norway, Schwitzerland and Tyrol should be Mountains. This areas had narrow passes, snow, and swamps together with high Mountains. I dont think a hill in summer Greece could stop an attacker in the same way as the Brenner pass during Winter
That said, the whole map and the movement system is insane. It takes several A&A years to drive trough Belgium and France with its highways and railways, because this small areas are cut in hundreds of territories, but luckily its the opposite in the wasteland since the Polar Express let us walk a inf unit through Russia and Canada in no time even if they have dirt roads only in the real world. Same at the sea. In real world at ship used two weeks to cross the Atlantic but it need a year in this game. Even a 1600 sailboat is faster than the AA cruiser