There is a ramp nearby the catapult. Immediately to the right of it is a small block laying horizonally, and another standing vertically on it. You have to squeeze the bomb directly between the ramp and the two blocks. Getting it perfectly up against the two blocks and 'catapulting' the vertical bar forward will hit the castle. I've only been able to get it once, and I had already taken a shot before. It's extremely frustrating. Good luck.
The flame won't destroy these levels. Dark materials gives you two chances to get a gold. You can easily use the large stones to detroy each tower at a time. House of Usher can be destroyed in one catapult with the purple bottle. If you don't have the purple bottle, you can use stones, or the bomb, it will just take a little strategy of alignment.
The hard part about Flintlock on the iPhone is that once you tap the screen the view moves very quickly to the trebuchet and you can't see the 1st obstacle which is the key to winning in one attempt. Use the bomb and shoot it towards the ground in front of the 1st obstacle. You want the bomb to have enough inertia to roll up and over the ramp and fall against the short vertical wall. The bomb ignites and blows the wall into the air, hits the castle wall and kills everyone. Took me about 30 attempts.
