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 entrance exams for any University of Magic worth its wands. The reanimated body does not have any intelligence or free will; however it can understand simple orders as "Hit this person" or "March this way". There is no limit on the count of raised bodies, nor on the time that one can keep a single body working. A good mage, straight from said University has enough power to keep a small army working all day, finding it no more exhausting than a short walk on the beach. After casting, a mage does not need to be concentrated on the 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 the 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 the magic does is 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 the 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 a good match for maybe a dozen heavy riders (think of knights), if they are on flat ground. The only way for this army to attack a 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 disappointment, his army was defeated in a fast manner by a small garrison of troops.
But how? Why was an army of fearless, tireless murderers who would kill a puppy without a second thought, who will obey any simple order, defeated so easily?
EDIT: Only a person who created a zombie can control it. The death of the controlling mage does not dispel the undeads, instead, they are going on with the 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 as nimble as humans. Yes, they can run, but slower cognitive functions cause them to fall easily. Still, outrunning a horde is not a very good option, as you will run out of breath long before you lose them.