Five keycards :D
Maybe I'm going to the wrong places, but I only found 2 key cards.
1. Red, somewhere in a random place
2. The green one in the room at the checkpoint.
When entering a room where a second-level key card is required. The screen goes off and the message "to be continued" appears.
oh, i think you are playing the "stable" version, yeah it's two cards, the new version has five, four obtainable and one craftable... xD
