I'm not sure this can be right. Surely the one unit incapable of resisting a charge, having been routed, is an artillery unit? For it to inflict huge casualties (more than 50% of strength in my example) seems absurd. Further, I've since played games where I have entirely surrounded artillery units and charged them from the front/rear etc. They have no nearby supporting units but inflict vast casualties on every unit that attacks them. This seems nonsensical
In addition, I have, on occasions, surrounded, completely, large numbers of enemy units in an attempt to achieve mass surrenders. Generally, the enemy units, even if routed, quite often just disappear (presumably having escaped) as the after battle report makes no mention of them having surrendered. Indeed, a certain amount of time after one is informed that the enemy has wavered and is fleeing the battle they disappear, again even if entirely surrounded. This seems bonkers to me.
Another point; one of my standard defences against the computer (which is generally impossible to attach as the advantage granted to it renders damage it can inflict deadly in open terrain) opponent is to create a laager-like line of units, entrenched so as to create what one hopes is an impregnable circular defence. I put weak units in the middle, in line, to chase a fleeing opponent and wait for the computer to exhaust its morale attacking my line. I have noticed that if I use forts to anchor that line, that the units ibn the forts are much more vulnerable than entrenched units. They (the fort-based units) seem very susceptible to charge (which seems silly) and cannot entrench as they're in a fort. Entrenchments seems to confer greater defensive benefits. Again, surely this can't be right?