I have been working on this guide quite from yesterday, and finally I have collected them all! If you have a problem with finding some of them, then you came to the right place! This guide will mostly be in pictures rather than text, because I thought it will be better to show you exact places than describing them.
Let's start with basic thing. For what do we need runes? To unlock main hidden level in the game. It's called "???" as it is one big secret. What it is? Well, we won't spoiler it here of course. Find out for yourself, or select the text (like you would've wanted to copy it) below!
And that's pretty much all. I knew a few of them, so I just ran towards them to make screenshots. Rest was pretty tricky to find, because, as you can see, it wasn't a button every time. Sometimes lever, sometimes tripwire or even a book! I will be releasing more Full Guides series about secrets, hidden levels and basic game mechanics. Stay tuned, and have fun with those runes!