Something is Amiss
Return to Hollow Bastion and you will find that the place is a bit overrun. Talk to Cloud then fight through the Borough to reach Merlin's House. You'll likely end up fighting a new Heartless along the way, the Bookmaster.
Monster - Bookmaster
Bookmasters are immune to all magic except for Reflect and Magnet. They use Fire, Blizzard, and Thunder from distance but using Reflect is a perfect remedy for that problem. A Bookmaster will get close if need be and has a Double Strike that will send Sora flying backwards. It can also use an upgraded form of this attack which is a bit more wild and has four total strikes.
Monster - Armored Knight
Armored Knights are the most represented enemy in the Battle of 1000 Heartless. They are true to their name, being very thin and humanoid while clad in black, grey, and silver armor. It has a spherical, pitch black head with glowing yellow eyes and two red spirals that decorate its visor. In combat the Armored Knight uses a Slash that is essentially a lunging attack with its sword. There is also the Spin Rush that is a horizontal spin, easy to Guard. In addition to these two moves is the Jump Rush, a leaping sword thrust that should be avoided easily by just moving. Lastly, you have the option of using the "Rising Sun" Reaction Command that will have Sora rise into the air and soar through the Armored Knight. You can use this command five times in a row if you need to, making this Reaction Command an effective crowd clearer.
Monster - Morning Star
A Morning Star stays true to its name and is essentially a large spherical Heartless with spikes. It uses several attacks to deal a lot damage but has a major weakness. The Iron Ball Punch is a quick left then right attack with its arms. The Spinning Mow-Down is where the Morning Star spins around with its maces. Spin Top is an attack that sees the Morning Star spin like a top as it pursues Sora. Lastly, is the Body Press move where the Morning Star leaps high into the air. Counter with "Bump" then "Meteor Strike" to deal a lot of damage.
From here head into the Bailey where you'll get a scene with the group from Final Fantasy X-2, Yuna, Rikku, and Paine. Ahead you'll fight another new enemy, the Armored Knight and at the bottom of the steps the Morning Star. Continue to the Restoration Site and through to the next area. Grab the puzzle piece Dawn #5 then loot the two visible treasure chests for the Castle Perimeter Map (6/13) and a Mythril Gem (7/13). Leap down and pick up the AP Boost (8/13) and head into the next area. You can't get the puzzle piece yet.
Monster - Surveillance Robot
A Surveillance Robot is a bronze-colored, funnel-shaped, mechanical Heartless with a diamond-shaped opening in its front, exposing its glowing, yellow eyes. Two black, rectangular guns are mounted on either side of its body. Each of these guns sports a silver, pyramidal spike on both of its ends. There is a silver ring that acts like a gyroscope around its base. In general, the Surveillance Robot is not a very difficult opponent and can prove useful with the "Sparkle Ray" Reaction Command. It won't damage the Surveillance Robot, but using "Snag" then "Sparkle Ray" will deal massive damage to all the other Heartless that it is pointed at. This move cannot be used on its own, however. The Surveillance Robot can use a Twin Laser to fire at a distance and strike Sora or use Mow Down, an attack where the Surveillance Robot spins in circles. Lastly, the Surveillance Robot can use a Charged Laser to fire multiple red lasers that deal moderate damage. In a group, be sure to leave these guys for last.
Navigating the Corridors
In the Corridors you'll fight a new enemy, the Surveillance Robot. At the first split turn left and acquire the Dark Crystal (11/13). Head all the way back to the left side of the map now and down a bit to a chest with an AP Boost (12/13) inside. With High Jump Lv2 you can leap from the staircase to grab Twilight #47 . Continue through the large area to the right side of the map where you will find two chests, a Mythril Stone (9/13) and Mythril Crystal (10/13). Exit at the south end to trigger a scene. Pick up the Skill Recipe (13/13) from the chest then speak with Leon. Enter the Computer Room and Sora starts mashing buttons, triggering another scene and taking the group into a new world.