Head to the right. Leave the floating legionary alone, you need him in a minute. Lead him over to the end of this area and make him drop a projectile so as to break the blocks. Kill him. Head onwards and grab the red potion from the pot. Go into the cannon and drop a potion. Do the same with the next cannon. You'll end up in the clouds. Hit the right-hand pot for twelve random items, then hit the left-hand pot for the key (if you hit it first the other pot disappears). Head right and unlock the door.
Head to the right. When you reach some solid wood you can kill the floating legionary if you're careful. The single column next to the big area of blocks contains a helmet. After this another floating legionary appears but you can't kill him yet. The very tall column soon after this contains a bag of gold. When you reach the first cannon jump onto it and get rid of the legionary. Head to the right either by jumping or by using the cannon. Use the cannon after that, jumping to the left as soon as you land to avoid sliding into the hole. Activate the cannon you've landed beside to reach the key area. There are two pots; the right-hand one spits out twelve random items while the left-hand one contains the key and will make the other pot disappear. Drop back down, head right, and go through the door.