Blue tower stamp???

[UaiOvelha]

I really want to understand what the blue tower stamp in the top right corner of the cards means when I open the codex. I've been trying to figure out the logic behind it, but nothing adds up. It's not using all levels, and it's not having played or added the card N times. I can't find any information about it anywhere.

[jafferduo]

There are 5 different stamps in this game.
You'll get these stamps when you use the cards beat Act 3 final boss on different 5 difficulties. Each difficulty has its own stamp.

[Sleepy Mill Studio]

Exactly, the stamps look the same as the illustration on the difficulty selection