Was playing the Middle Ages Scenario. Was Sweden, and had conquered Norway. A barbarian invasion was spotted at the northeast corner of the Norwegian peninsula (the edge of the map nearest the water).

I marched some berserkers up to attack, and was able to occupy the edge square next to them, but was unable to attack. They also appeared unable to attack me.

But they did keep my citizens from working that square.

Not sure if this is Conquests, scenario specific, or just a Civ thing...

This also happened to me in that scenario, seems units can't go on the last row of tiles...