Castle of Elite - Official game guide|
Introduction - Basics - Solutions - Secrets - Trivia
When the guide tells you to place a brick symbol on a certain square, it means you should aim the mouse cursor on that square.
3-9: Trundling snakes
A: Get this brick and place it on B.
C: Get this brick and place it on D. Hit the blue switch. You can now get all the coins and reach the goal, because all the Trundlers will be trapped on the right side of the level.