Does this aid gameplay somehow?

I don't think so, which would make it bad.  .

I do think the "small, medium, large" like SC is ideal, as it's simple.  But I just don't see the need in Demgiod.  It seems to work fine to me without them.  There are no damage-types in an FPS for good reason.  I think for Demigod it's the same reasons.

The only special attack type i think is skills.

Skills should be capped 33% armor reduction.  So a skill can't do less than 2/3rds dmg no matter how much armor someone has.

So a rook with 60% damage reduction would still take 2/3rds damage from skills.  But if someone has less than 33% armor, they'd take that much.

CURRENTLY skills ignored armor, but I was told that this wasn't intended and will be changed.

Of course there needs to be some line on armor that says it's capped at 33% vs. skills.

Special armor ignoring skills are fine yeah, but they should be the exception, not the rule, and clearly marked.

I never noticed doing less damage to any units.

The armor/damage types probably only effect creeps fighting each other, but not Demigods, which works fine that way to have creeps RPS each other but not Demigods.  Or they just do nothing.

Currently the only damage type is that siege weapons do double damage to buildings, which makes sense and works well. Let's leave it that way. Everything else is at 1.00000. So you are mistaken on that count. However, they have a commented out section which may contain types they're thinking about. Here they are, bear in mind they aren't implemented and there's no note about what they're there for. They may be an old version from before they abandoned the idea:

#ArmorTypes = {
#    Unarmored = {
#        Normal = 1.0,
#        Hero = 1.0,
#        SpellFire = 1.0,
#        Spell = 1.0,
#        Light = 1.0,
#        Arrow = 1.25,
#        Artillery = 1.0,
#    },
#    Normal = {
#        Normal = 1.25,
#        Hero = 1.0,
#        SpellFire = 1.0,
#        Spell = 1.0,
#        Light = 1.0,
#        Arrow = 1.0,
#        Artillery = 1.0,
#    },
#    Heavy = {
#        Normal = 1.0,
#        Hero = 1.0,
#        SpellFire = 1.0,
#        Spell = 1.0,
#        Light = 2.0,
#        Arrow = 0.5,
#        Artillery = 2.0,
#    },
#    Structure = {
#        Normal = 0.75,
#        Hero = 1.0,
#        SpellFire = 1.5,
#        Spell = 1.5,
#        Light = 1.0,
#        Arrow = 1.0,
#        Artillery = 2.0,
#    },
#    Demigod = {
#        Normal = 1.0,
#        Hero = 1.0,
#        SpellFire = 1.0,
#        Spell = 1.0,
#        Light = 1.0,
#        Arrow = 0.5,
#        Artillery = 1.0,
#    },
#}

It looks relatively simple. Lighter attacks are less effective against heavy units and buildings, against which the heavier attacks do better.

For catapultasaurs yes.

Sadly though.. arrows and stuff are homing.   Which kinda bothers me..  I like when things are more powerful but dodgable.

Dodging stuff is cool, but a decent player just goes and dodges like 30-40% of your arrows + fetch some Evade equip and bam you'll deal 0.0000% damage to target melee which runs up to you and whacks ya face off.

I'd say, artillery should be like they're now, and Arrows shall be homing else its really hard to hit anything. I mean, honestly, the first time a catapulasaurus hit me was also the last one. I noticed they're not homing, and dodge even with a rook, without any bonus speed, every single shot they fired at me <.< Ok the projectile is not that fast but.. dah, i simply dont like it. This is not a shooter where i've to aim, i mean melee also just walks up to you and auto-attacks you.

Else you had to implement something, that ranged players can aim where they shoot. But else they can only give an attack-command and -hope- it hits you. That would be just unfair.

So long, Aspartem 

I think there should be weaknesses against magic, and against pysical damages. This adds variety, and it's simple.

When you choose some equipment, a very high defense armor should not protect you very well against a rain of fire, but very well against minotaurs or Rook attacks. If you want protection from magic, you buy appropriate stuff.

It's extremely basic, I wouldn't understand if they don't do something like this. For buildings also: magic shouldn't deal much damages against them.

always*

And that's before armor modifier, of course.