Hollow Bastion
You'll start out with no Keyblade, so run or dodgeroll across to the Castle Gates, they'll be closed so save and then go to the lift round the bottom right of the map. When down, go to the right and interact with the switch, it'll turn off and turn on the red switch which you have to get to before time runs out. Now go across the lock-by-Blox thing and then turn right and touch the blue switch, now blocks will fall down and go back up and some will hurt you others won't, so make sure to reach the red switch opposite the blue in time. After you press the red switch, a third switch will open up in the middle, press it.
The doors must've opened by now, go back up with the lift and get through the Castle Gates. Save before going through the entrance, you're now trapped with Heartless, just keep running around and dodgerolling... GOOFY! You'll play a small while with Goofy fighting for you, after the cutscenes go up to the Lift Stop... wait, the door's closed, you'll need to get all Emblem Pieces to open it.
Go to the door on the far right when facing the exit to the Castle Gates, enter and get the Emblem Piece off the table. Go back out the door... DONALD! Go after him, you have two choices out here, you can either explore with Goofy or get Donald. I'll go with the previous, so have him break every block in the area and then attack enemies, then interact with the switch on top of the pile of blocks, you'll have to go over the midway platform to get to the balcony on the other side, so do so. Then go across to the copper platform and get the Emblem Piece, now go back to the lift to get Donald.
First, break all the Blox close to you, then go through to the side with the three switches and let him break the wall of Blox, by now you can see Donald on the other side, but he's trapped. First, interact with the blue switch across the area, forget the Blox Bugs. Have Goofy break the blocks, then touch the lava-like Blox to blow up, you'll get Blizzaga and be able to interact with the red switch. Interact with the other blue switch and have Goofy break the wall for you, on the other side there are more lava-like Blox, touch them to break the rest, then interact with the red switch. Donald is free! He can activate the switch in the Library!
After getting Donald, go to the other end and get the Emblem Piece as well as any other blocks. Use the new red switch to get a lift that is directly beside the Castle Gates.