Now that you have all 4 flints, it's time to move on. You may have noticed a pool of water in the center of the fortress if you wandered out that way. There is an alter at each corner. Lighting each alter (stand beside it and tap it) will start a fire. Once all 4 alters are lit, the pool will drain. and there you will see a large key at the bottom of the pool. That key unlocks the middle door directly to the north of the pools ladder. That's where you go next. Save your game first!