I have never had a General dissapear on the campaign after routing on in a field battle. It might be a fluke. Then again I always reload battles I start and lose. My main generals almost always have flawless records.

Also, a bit of advice for your siege tactics: never try to take the gate. Take the surrounding walls. When taking walls the gaps are much larger so enemy units have to spread wider than they would if they were defending a gate. Also always build 2 rams and attack from opposite sides (I usually attack each section of wall next to the gate). This splits the enemy and leaves your troops (who I hope are numerically or statistically superior) with less men to deal with at once. Sometimes the enemy completely ignores on gap and is left vulnerable to flanking attacks.