League of Legends: Turrets

One of the new masteries added to League of Legends in season 2 is called Siege Commander. It reduces the armour of nearby enemy turrets by 10. I was wondering about how good this actually was (I have it in my Warwick spec) and tried to take a look at the armour of the enemy turrets in a game as I was walking up to them. The turret went from 250 armour to 50 armour as I walked up to it which was really confusing. It says it knocks 10 armour off! Why did it lose 200? Clearly some further investigation is required...

I did some searching and found that turrets actually have all kinds of weird rules surrounding them. I'd always just assumed they had static stats (possibly that scale as time progressed) and just shot for sensible damage. It turns out there are weird rules for how much damage they do and how much damage they take.

In terms of how much damage they do it seems that every consecutive shot on a single target does an extra 22.5% damage. So getting hit once will hurt but getting hit for a bunch of shots in a row is going to destroy you. Turrets also have 20% armor penetration and do only 94% damage to champions. 

Defensively if there are no enemy minions within range of the turret it gets x3.88 ac and x2.47 mr. Presumably this is to make it harder to 'back door' without a creep wave present and is almost certainly why the tower lost 200 ac in my test. 

And perhaps most interestingly... The damage a champion does to a turret is either her attack damage (the same as with every single other thing you can auto-attack) or 40% of her ability power. This blew my mind. I've always operated under the assumption that casters are terrible at burning down turrets! I popped in a quick replay from a game yesterday where I was Veigar. I had 109 ad and 690 ap. I was cracking a turret, post ac, for almost 200. Assuming the 40% number is right that means I do 2.5 times as much damage to a turret as I was expecting I would. Not bad! A true physical attacker is still going to do more since they're apt to build some sort of attack speed items and I'm really not with Veigar. (Or am I...)