For all you rookies out there here are some of my own experiences
For sieges the best thing to do after you've destroyed the ranged units, use an order made to cast a displacement spell on one of the enemies near the wall so that he comes adjacent to the wall, now he's in range of your range attackers and since your armies are way way back from the gate the computer wont even bother to come out, and even in multiplayer it works fine, but you do get off a few attacks before people realize it
Secondly while against any difficult opponent get a high level spell caster to dulf a potion of fire resistance and armageddon, its ok if you have fire elementalsor efreets , i practically won whole of HOMM 3 with it, and am getting away with it in this too !
I'll keep posting more here and then !