And (almost by accident), it makes it difficult for players to capitalize on one winning strategy for putting a rune brigade on the field that is sure to win. What i mean by this is that it "should" be hard to copy another player's rune army because of the customization level in the game (classes being a big part of that).
Only Immortal Runes have classes (creatures are just creatures). Immortals start at level 1 and can be either a spellcaster or fighter.
If you choose to be a spellcaster when you get to level 3 you can choose to then become a magician, witch, or priest. A fighter at level 3 can become a warrior, scout, or rogue. This is where things first start to diversify.
Classes continue to branch out after this. For example, a Magician at level 7 can become an elementalist, wizard, or warlock. A warlock at level 12 becomes a channeler or geometer. At level 18 the channeler becomes a conjurer and level 25 that conjurer becomes an astral mage. All class progression basically works like this, depending on the choices you make.
Here is a visual look Archmage class (spellcaster, magician, wizard, evoker, invoker, and archmage.
You can various abilities when you gain a new class. This can be anything from a discipline, to a weapon skill, to something unique. For example, the Magician gets the educated discipline, the wizard gets the shield discipline, etc.
Spellcasters also get a new spell. The Magician gets Summon Shield, the Wizard gets Whirlwind, etc.
One idea I came up with for Immortal Kingdoms is accumulating classes. I'm not sure I have seen this in other games but it may be found elsewhere.
Accumulating classes means that when you acquire a new class (levels 3, 7 , 12, 18, and 25) you also start a new class. Its kind of like multi classing in Dungeons and Dragons but on a mass scale (everyone does it and you get several classes).
So, if you start as a fighter and gain level 3 and choose to be a warrior you also get to start over at 1st level with a 2nd class.
For example, the Immortal above might choose to start over as a spellcaster. He would then become a level 3 Warrior and level 1 spellcaster combo. And he would gain the abilities of a warrior and level 1 spellcaster.
So when a character reaches level 25 he would have 6 classes...and have all the abilities of those classes.
The only restriction to the classes you can choose is alignment. Some classes have alignment restrictions. Like Bishops have to be good, Pirates chaotic, and Assasin's evil. When selecting new classes you will be given a list of the classes available to your alignment (as well as when you advance a level).
The one negative to all these new classes is that you also get the lower ability scores of these lower level classes.
What does this mean? Well, when you gain a level you get increased stats (strength, defense, etc) based on your class. So, the higher the class level the more you increase in your stats. But this is also class dependent (warriors get more attack, defense while a wizard gets more intelligence, wisdom, etc).
How does this work exactly? You average the classes stat rolls.
For example, if you are a level 7 wizard you get 5d4+6 intelligence per level. But since you this Immortal is also a Magician that gets 4d4+4 per level and a spellcaster that gets 2d4+4 per level you have to average it all out.
So you might get 16 as a wizard, 12 as a magician, and 8 as a spellcaster. This totals 36. Divide that by 3 = 12. So you would get 12 intelligence for that level (say when you advance from level 7 to 8).
Anyway, the basic idea is that you are both helped and hindered by the new class. But even with this minor set back the Immortals will become more powerful than creatures at higher levels because of their versatility and many unique powers. Note: Creatures also will have higher health point totals than Immortals.
You don't have to actually know all this but it is important to understand the basic concept.