You've fought through many different lands, defeated tons of Dark Matter's minions, and now you're at the evil being's doorstep. You will have to charge straight into Dark Matter's fiendish castle to challenge the dark force itself. Dark Matter is well prepared for your arrival though, packing the stronghold full of his remaining army and assorted obstacles. All but one of your Animal Friends can be found within the castle, which is Nago the Cat. Bring him from another stage and clear this difficult level to reunite him with his lover!
Assault on the fortress
Note: Before you start the level, note that you need to clear this stage with Nago to get the Heart Star. However, Nago isn't present at any point throughout the level, so you must find him from another stage before you enter this.
You're now inside Dark Matter's stronghold, so the difficulty is going to be higher than ever. The fortress's first line of defense are an army of Bukiset. Fortunately, this means you are free to choose which copy ability you want. Watch out for the Bukiset with ranged attacks, as they can make traversing the area's corridors a risky task. Once past the Bukiset, you get to choose between taking Rick or Kine with you, but you would still want to keep Nago if you're going for the Heart Star. Ignore the two animals and proceed to the next area, which contains tricky circles of Gordos that require good timing to avoid. Then, you must run through a corridor with tons of Glunks. These Glunks launch their projectiles at different intervals, so you can't just run past them in one go.
The next couple of rooms have pretty simple themes. You must ascend up a staircase as Sasuke fall from gaps above, not a hard challenge by any means. Then, you must cross a bridge over several pots that spew out Wapods, which are another easy enemy to defeat. After that, you must cross a stretch of collapsing blocks over some Ticks. There are collectible stars just above each Tick, but the damage you'll likely take from getting them isn't worth it. Past that room is a reappearance of Madoo. Here, let it break the blocks at the center of the floor to reach the stars and healing drinks below. You can't make Madoo respawn if you beat it, so don't quickly kill it when it appears. Next, you must contend with two Gansan rolling at you slowly, which can be dodged with ease by simply jumping up the platforms to the door. Going through the door leads you to another Animal Friend rest area with Chuchu and Pitch. Ignore them if you still want the Heart Star and move on.
The stronghold's tricky passages
The next area is a simple hallway with falling stone blocks as obstacles. There's no point in rushing, just let each block fall and cross them when it is safe. The large stone block in the middle is impossible to cross safely unless you break the Star Blocks below it. After this room is a hallway with Poppy Bros. Jr. bouncing in small gaps in the floor, then a similar room with Togezo instead, then a wider space with a few Scarfies. None of these enemies should pose any challenge to you by this point. After the Scarfy chamber, immediately start breaking the Star Blocks to the right as fast as you can. If you manage to make it to the other side before the boulder above you does the same, you get to collect a nice 1-Up. After that is a small room with some Chillies, whose Ice ability would be useful later. Next up is a chamber full of Shotzos on both ends. The left side of this room is easier to pass as you can destroy the Star Blocks to disarm the Shotzo, but it is much harder to do so at the right portion, so simply avoid all of the Shotzos at the right end and go straight for the door. Go through the next door and you will end up in yet another rest area, this time with Coo and Kine.
After the Animal Friend area is a small room with a 1-Up and a single Yaban. Take its hint and destroy the block just above the 1-Up with a direct attack to reach it. Next, you will be in a room with tons of collectible stars and some Propellers in a sealed off area below. The Propellers are trapped there for a set period, after which they start homing in on you, so be sure to collect the stars quickly if you don't want any trouble. The next room is a simple chamber with a couple of Mariels that you should have no trouble defeating. Then, you'll end up in a room with some Sparkies and Bobbin. Wait for the Bobbin to leap out of the water before fighting them, and be careful of the Sparkies' sparks passing through the walls. After the waterlogged chamber is another bonus room with a single 1-Up. If you have the Ice ability, you can break through the blocks here to get the 1-Up, but watch out for the Peloo hidden in the middle of the room. Past that is an area full of Pteran. Defeat each of them one by one while you're still in the alcove you start out in. In the next room you can get a rare sighting of a Batamon. After that, you will end up in another rest area once again, where you can join with Rick or Chuchu.
Heart Star: Nago's romantic reunion
Past what must be your fourth rest area, there is another small theme room with three Galbo. After that is an odd hallway with spinning bars that you've seen previously. You can't destroy them here, so watch the tiles on the floor to identify their range and move past each as they're spinning into the ground. Next is a short ascent featuring a trio of Nidoo very obviously disguised as doors. Continue climbing up a winding area full of Keke and you'll reach some familiar drawings. Like you've seen back in Cloudy Park, these paintings are actually real enemies, and every one of them will come out of the walls to attack you. After that is a treacherous pool with some Joe that you must brave to get a bunch of small stars. Up next is a hallway with many Kapar, then a bonus room with some healing drinks and stars. Watch out for the Peran hiding in the walls as you collect the drinks and stars. After that room is a mostly vertical pathway populated by Mumbies, as well as some health drinks. Go through the door at the top to find yourself in another vertical room, which contains green goo that will automatically shoot you to the very top. At the very top, you will finally reach the exit door.
Living in the castle is Nago's girlfriend Shiro. If you manage to bring Nago through the fortress's gauntlet of enemies, she will reward you with your penultimate Heart Star.