There are few places in the game where placing 1-2 units will block off a pass completely. Placing a unit near a city also doesn't work because the enemy just goes around and attacks the city. So, basically, I end up having to go on the offensive even though my enemy is the attacked and I am defending my province! While attacking a beseiging army from behind is much better than sallying forth from the city, I find it frustrating that I am the one that has to "attack" in order to "defend". Is there a way to make the enemy attack a unit instead of just beseiging my settlement and waiting for it to fall or for me to attack?