2. There is a penalty for landing in a defended beach. This would need to be high enough that even a townsland would have a good chance of fending off an amphibious assault, or at least inflicting heavy casualties on the attacking force.
This is the main point in my opinion. If the townsland is undefended, then an amphibious gives you complete surprise. In most cases, the mobile army will be far and not able to react in time. Landing an army right next to the capital without haviong to go through a main mobile army would be devastating.
However landing half a broken army would not be very devastating at all.
I expect the penalty to be strong enough that in almost all cases the landing army will want to land in an undefended rural. It does change the strategic thinking, but it's not so devastating to the cities.
