Quote Originally Posted by Lord Fluffy
Unless you're fighting the Mongols where almost all their generals are night fighters.
Which, let's face it, is the only time you really need to split up enemy armies anyway - to be able to duke it out on the field with just the one Mongol army at a time, rather than four at once.

Otherwise, I find the enemy reinforcements usually end up working to my advantage; how many times have I captured the enemy citadel without a fight because I drew the garrison out as reinforcements and annihilated them in the field?

That's an important point actually - if the enemy is attacking with two or more armies, you should always attack the first army immediately to defeat them in detail before they can unite even if you are technically on the defensive, unless you really do have a truly unassailable defensive position.