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.
I'm not 100% sure about it, but: Each wave increases Armor by +1 Special wave: Armored or how its called +10 for the round Bonus rounds are like normal waves, without counter and drops. So it will increase like that too. Special properties of the Creeps is hidden as well. Challange and Horseman might be different ;) like +25 armor or something or they just have a big fat life bonus... One possibility could also be, that they start with 5/10/15 armor or whatever +1/wave. But thats just a guess. The thing with +1/wave and +10 for Armored Creeps was stated somewhere by Andy when i was rather new... Cant point you to it though... Might also be my memory is messing up ;)