Once you have control, enter the palace to the right; the portcullis will shut behind you. Continue right, and the mouse will direct you through the wrong doorway.
In the room with the spider, try pulling the doorknob 3 times, and it will fly off to the left. Head left twice and pick it up. Return right twice and throw the knob at the spider to get it to run upwards (this will scare away the mouse). Go left twice again, then climb up the stile to the left. Pick up the sword, then use it on the rope. Pick up the shirt ghost and the feather, then head right. Use your gun to shoot the stalagmite, then continue up both ramps.
Move the orange switch down and leave the pink switch up. Go left, then climb up and go right. Grab the blue glob from the creature's nose, then tickle the nose twice with your feather.
Return left and go back down and right. Insert the blue glob into the blank space. Now move the orange switch up, the blue switch down, and leave the pink switch up. Go left, then climb up and go left. Grab the little stone, then return right and climb down again. Go right past the levers and down both ramps. Insert the little stone into the hole, then continue left. Jump down the slope.
Quickly step on the switch, then head to the left. Quickly grab the rope.
Walk left to the safe place. Use the t-shirt ghost on yourself, then head to the left. Continue left until you see the chess board, then smash it when the ghost on the left is snoozing. Now return just to the right and take the match from the table. Now head left until you see a big ghost. Use the match on it, then continue left.
Quickly press the switch near the top of the door.
Pick up the golden key, then select option 1 when talking to the mouse. Quickly press the switch behind you, then press it again. Now pick up the golden key again.
Use the golden key on the door. Once inside, take the diamond.