This should answer your question:
https://mazebert.com/2014/05/18/new-armor-calculation/
A useful trick to know is that you can search any website using google. I found that post by entering "site:mazebert.com armor" in google.
It does answer part of the question. Thank you, I did try but my error was to look for "defense" and not "armor", my bad ^^.
One question remain unanswered though : how does creep armor increase ? I mean, how do I know how much armor a creep starts with ? I'm guessing it is linked to the level, but then (1) what is the formula ? and (2) how does it behave in bonus levels, when there is no more level info ?
thanks again for the previous answer !
I think It is 1 per round number, plus some bonus for armored rounds. Not 100% sure though. The bonus round is made of rounds, too. They just come right one after another and there is no round counter anymore. I think the armor-increase formula stays the same.
Thanks for your answer, that is what I suspect too, maybe a developer will shed some certainty light on those assumptions :). In the mean time I'll put my money on your theory, thanks again.
