In the campaign Vengeful Throne, map 3 seems to be unbeatable.
At the start, the opposing forces are located on two different shores of a river that crosses the whole map.
The bridges that cross the river are neutral. So what happens is that the enemy AI destroys all bridges in turn 1 and from that moment on, all parties can live their lives in peace on their respective sides of the river and no one can bother the other side.
But defeating the map has become impossible
And just in case somebody wonders: No, there are no builders or factories of any kind that would permit rebuilding those bridges.
Revenant