In Kingdom Hearts how do you get the heart in Traverse town

After unlocking the red trinity, return to Traverse town. Go to the gizmo shop in district two. Climb to the roof. There you will find that a few boxes were moved and you have gained access to a red trinity. Obtain the trinity and you will find a bell. Ring the bell. Keep ringing the bell until the keyhole can be seen. before you can lock the keyhole, you will have to defeat the Knight armour heartless.(Sorry I forgot his name)

P.S. I am unsure if doing this part will affect the part above:
Before doing the part above, you may have to get the gizmo shop running again. To do that use the lightning spell on the exposed wires in district three(Near the door to district one). Doing this will supply power to the gizmo shop. WHen you go to the gizmo shop, lifts will be activated on the walls of the shop. Use them and press all the buttons that are near the ceiling. Once again I am unsure if this part is needed in oreder to ring the bell