All the answers here! The main goal in 100 Doors is to escape the room. Using all the possibilities of the device you need to open each of 100 doors and move the doors to the next Level.
Search for hidden objects, solve puzzles and use an items to do this!
Tap the lantern to light up parts of the screen.
Wait until the light is showing the bottom left part of the screen and tap the chest.
Wait for it to go back to the same spot and grab the key that was in the chest.