Angel:Angels are the best unupgraded level 7 creatures in the game. They are also the most expensive unupgraded level 7 creatures (the Portal of Glory where Angels can be recruited is quite affordable though) but as soon as you look at their skills and stats one must realize why this is so. Their speed is really high, they do a nice (very very nice) steady damage, increase the army's Morale by 1, have splendid Attack- and Defense skills and they have tonns of healthpoints. Angels are true leaders of Castle troops and very hot to blend into armies of other creature types as well.
Archangel:Archangels are among the best creatures in the game. Their Attack- and Defense skill is only matched by some of the neutral dragons, and there exist only 2 faster units. (the Azure Dragon and the Phoenix) These things among with the great things inherited from the Angels and their stunning ability to Resurrect makes Archangels the best allied (not neutral) offensive unit. The only drawbacks are the huge cost in gold and the number of healthpoints. (a little bit lower than some of the other upgraded level 7 creatures) When you have lots of them, it becomes useful to split them up. That way you can resurrect multiple targets or resurrect the one stack of Archangels with the other. There is also a combo that involves Archangels, check the 'Spells section' under Clone if you want to find out more.
Archer:Archers are useful because they are ranged attackers, but you should upgrade them as soon as you can. They are below average unupgraded level 2 creatures and can not compete with other level 2 ranged attackers.
Cavalier:Cavaliers are a true asset to any army, they have a lot of healthpoints and deal large amounts of damage. The Jousting bonus increases the amount of damage they deal by 5% per hex they move. The only problem is the cost of the Training Grounds where Cavaliers are trained. It costs 20 wood and another 10 to upgrade it. Cavaliers themselves are expensive in gold but definitely worth the cost. Still, their combat power increases a lot when you upgrade them.
Champion:Champions are great offensive units. They are very fast, which is nice for their Jousting bonus too. When Champions can run for their full 9 hexes, they deal 45% more damage than they normally would upon striking. They are somewhat expensive units, but they are really good for any army type and their enhanced speed and damage makes them much more offensive than Cavaliers.
Crusader:Without their special ability, Crusaders would be average level 4 creatures. The fact that they strike twice makes them formidable troops. They are useful in any phase of the game. Though their speed is quite slow, some Haste or a Prayer can do miracles on them.
Griffin:Griffins are average unupgraded level 3 creatures. They are quite useful during the early game and when upgraded, their speed is of a great strategic importance during the early game. Griffins themselves cost less than most other level 3 creatures, what makes them very interesting when you are low on cash. Their flying ability is also very useful to Castle strategy.
Halberdier:Halberdiers are undoubtably the best upgraded level 1 creatures. Because of their large amount of healthpoints and their nice Attack and Defense ratings they can still matter during the mid game and they remain good to guard a town during the entire game as long as you have them in large enough numbers.
Marksman:Marsmen are extremely valuable ranged attackers in a Castle army. They deal considerable amounts of damage, they shoot twice, plus they grow fast so you can create large stacks of them. Though they are among the best upgraded level 2 creatures and their cost in gold is low, Marksmen have a number of serious drawbacks as well. They are relatively slow and they have a bad Defense skill and a very low health. This makes them vulnerable units too. Never leave them open for melee attacks but provide them with some protection.
Monk:Monks are great ranged attackers but are even more vulnerable than Archers or Marksmen. Their low Defense skill and relatively low health (among other unupgraded level 5 creatures) is a serious problem that is partially solved by upgrading them.
Pikeman:Pikemen are among the best unupgraded level 1 creatures. They are somewhat slow but they have a lot of health and a descent Defense skill. When upgraded, Pikemen gain speed and remain useful during later stages of the game.
Royal Griffin:Royal Griffins are not much more powerful than simple Griffins and they are a bit more pricy too, but their speed is amazing and a great advantage to spellcasters. In large maps, these beasts will eventually fail to remain useful in an offensive army unless you still have large amounts of them left. But since Royal Griffins are such offensive creatures, you will probably have lost many of them in early battles.
Swordsman:Swordsmen are average unupgraded level 4 creatures. They are not bad, but they should be upgraded as soon as possible because that really makes them twice as good.
Zealot:Zealots do not cost much more than Monks, but they are quite a lot better. They have a better Defense skill and they suffer no melee penalty, what makes them considerably more dangerous. Zealots' greater speed also makes them more valuable and they have twice as many shots as their brethren the Monks.