In my custom fantasy world, raising the dead is really simple. Before the Great Act of Beautiful Death raising a body and keeping it alive(undead?) for at least a week was a part of exams before being admitted to any University of Magic worth its wands. Body does not have any intelligence or free will, however can understand simple orders as "Hit this person" or "March this way". There is no limit on count of raised bodies, nor on time one can keep single body working. Good mage, straight from said University has enough power to keep a small army working all day, and he would find it no more exhausting than a short walk on the beach. After casting, mage does not need to be concentrated on spell - Plug(magical energy)&Play(Murder)
Zombie have no feelings, no morale, and feel no hunger. The sugars/ATP in muscles is substituted with magical unobtainium. However, oxygen is still required, so no underwater walking. After magic in them vanishes, the body will start rotting, but nothing happens while it is animated. The body can be killed in any way that would kill a normal person, because all magic does is to force blood flow and send signals to body parts. However, the body will move for as long as there is enough blood to keep muscles oxygenated. And even without any blood, zombie will still keep attacking for a few minutes due to anoxyreaction of unobtainium(which destroys it in process). Still, they are more durable than any human warrior, and can swing heavy weapons all day without becoming exhausted.
From this short starting, you can think that any of those mages could easily overwhelm any human army. It is however not true.
An army of 1000 undeads is good match for maybe a dozen heavy riders (think of knights), if they are on flat ground. Only way for this army to attack the city is to flow over dead bodies of their fallen comrades, so actually as long as defenders have enough arrows no zombie can get in.
One Very Evil Black Mage, very intelligent but detached from society, has closed himself in his tower for ten years, and came up with plan to dominate the kingdom - he will have an army of said 1000 Zombies and attack the capitol! To much of his dissapointment, his army was defeated in fast manner by small garrison of troops.
But how? Why army of fearless, tireless murderers who would murder a puppy without a second thought, who will obey any simple order was defeated so easily?
EDIT: Only a person who created a zombie can control it. Death of mage does not dispel undeads, instead, they are going on with last order they were given.
EDIT2: No city has any undeads, because of GABD. Necromancy is forbidden and nobody can do that, unless he wants to be hanged. If he is lucky, that is. It is similiar to biological weaponry in our world - it is forbidden and no lawful state will create it, but terrorists may. So no fighting fire with fire.
EDIT3: Zombies are not so nimble as humans. Yes, they can run, but slower cognitive functions are causing them to fall easily. Still, outrunning a horde is not very good options, as you will run out of breath long before you lose them.