There is at least 1 outside all boss rooms. The second chest has a chance to be this type or an Iron Chest.
The simple chest is the only chest facing left.
Becomes increasingly common as more blueprints have been collected; drops in moderate to high amounts
Common, but with decreasing commonality as more blueprints in the area have been collected
Occasionally replaces simple chests outside of boss rooms.
The set of blueprints given by the chests only expands in later areas, e.g. chests in the Forest also contain blueprints available in the Castle, chests from the Tower contain blueprints from the Forest and Castle, and chests in the Dungeon can contain all blueprints. This means it is entirely possible to finish earlier sets when opening chests in later areas.
Take no damage: Varies greatly in the enemies/obstacles in the path, but getting to the end without taking a hit is all that's required. Sometimes flight will allow you to navigate the spike maze, other times dashing quickly, or picking off enemies is best. Most of them can be easily bypassed by Assassin's Mist Form.
Defeat all enemies: A lot of powerful enemies are waiting to kill you. In order to unlock the chest, you have to kill everything, not counting traps vulnerable only to damage return.
No looking: Your character must not face the chest. You can use dash to move without rotating or attack and move mid-swing (if you have Flexibility you will turn mid-swing so you must use dash).
No jump: You must gain access to the chest without jumping. Although it often appears to require multiple dash/fly runes, the ones with multiple platforms can be conquered by simply standing on the edge of the platforms to allow a singular dash rune to get you across. Also, flying as the Dragon class does not count as a jump, allowing you to get the chest easily.
Reach me in "X": Reach the chest before X seconds. Use dash to move faster, and be certain to not get hit/stun-locked.
No fighting: Simply get to the chest without attacking - you're allowed to take damage. Note that getting hit while wearing equipment and/or runes with Retaliation will count as an attack.