Thanks, NDLVR.
Hi gamegirl, in addition to what NDLVR advises, be sure that you are outside, and in an open space well clear of the castle and away from any paths.
Then, place your cursor to the left of, and just slightly below, the halfway mark on the lever control. Then click-hold, and drag your cursor up a little. The lever moves straight to halfway.
If you just double-click at that point, although the lever moves up without stopping at the quarter-mark, the game doesn't like that method, and...kaboom.

and let us know how you get on.