1500 M is the fourth subchapter of Chapter 7: The Summit in Celeste. The subchapter brings back the mechanics of Chapter 3 and utilizes them in new ways. The Cassette to unlock Chapter 7's B-Side can be obtained in this subchapter. In the final room of the subchapter, Madeline and Badeline will talk to each other while being boosted up from the last purple orb, then combine back together to enter the next subchapter.
Gameplay[]
Dust bunnies and sludge variants will make up most of the challenges in this subchapter, with careful stalling inbetween moving sludges using both mid-air dashes being a key strategy. This will be a difficulty spike from the previous subchapters as many rooms can be very hard without good knowledge of the proper timing windows to get through them. In the fourth room of the subchapter, a single key token floats within 4 moving sludges in a rhombus formation. Wait at the left ledge, watching for when a moving sludge goes from the bottom diagonally to the left. Right when this moving sludge goes back to the bottom, jump forward and dash diagonally behind it into the key token, then keep moving right nudging yourself forward ever so slightly through the other moving sludges. In the room right after it, the middle section should be done with only straight right dashes to consistently graze over the dust bunnies. The rest of that room and the two after it will just be getting good timings past moving sludges and/or circling sludges, so watch until you see the right time to fall down and dash forward. The ninth room of the subchapter is a large pool of dust bunnies with a spring attached to a platform above and a moving sludge on the right. Wait for this moving sludge to be on either side (aka, when it isn't currently moving) to dash straight up into the spring and hold no inputs, then dash diagonally up-right to safely fall into the gap.
The gold piece of the Crystal Heart of this chapter is located in a room hidden through a passage at the right end of the seventh room of the subchapter. This room will merely require some decent timing dashing inbetween moving sludges to eventually get to the gold piece.
The Cassette room is located just above the start of the fifth room of the subchapter through a hole in the ceiling. The Cassette collection can actually be tricky here, but the main trick is stalling in the air with both of your dashes to wait out the cassette block alternation.
Strawberries[]
There are 8 Strawberries found in this subchapter.
Berry 1
Found in the first room of the subchapter, this Strawberry is hovering on the right side out of the way of the main path forward. It's possible to jump and then dash twice to the right quickly into the dash crystal, then immediately dash straight up twice and grab onto the wall next to the berry. After collecting the berry and waiting for the dash crystal to reappear, fall down when the time is right and dash twice to the left in quick succession right after reclaiming your dashes.
Berry 2
By going into the gap in the floor of the third room of the subchapter, an extremely secretive area is uncovered. In this room below, this Strawberry is found. From the right side, you can just jump off the ledge and dash to the left when the moving sludge won't be in your way to reach the sinking platform. Jump and dash to the left off of this platform when the circling sludge and moving sludge are not in the way and you can get to the berry. Do the same back to the right with your remaining one dash and repeat again to get back to the ledge.
Berry 3
By going into the gap in the floor from the previous Strawberry's room, another secret room is accessed. This room has many key tokens surrounded by many platforms with sludge-spawning tiles on them and no floor. There are many ways to go about collecting them all but the easiest will start with the player standing on the disappearing platform and dashing left to climb the platform you arrive at. Jump left over the sludge and then dash diagonally up-left BEFORE touching the dash crystal to get your dashes back after doing the dash. Do another dash leftward for safety to collect the first key token, then dash over to the right to climb up the bottom platform. Jump over this platform to the right to collect another key token and grab onto the platform above. Stamina will be running low by now, so climb a bit and jump to the left to the other platform, then wall jump back and jump into the dash crystal. From here, fall down inbetween the gap of sludge from the lowest platform and the first platform and keep moving right, then dash straight to the right into the third key token and then straight up. Climb up the right side of the platform and jump to the last key token, then wait for the key-activated platform to go down and land on it to then go to the Strawberry.
Berry 4
Found in the fifth room of the subchapter, this Strawberry is in the middle section of the room at the bottom around many dust bunnies. There's a bridge to stand on to the left of it, so stand there and face right. Dash right into the berry and then quickly move left and dash diagonally up-left just before falling to the bottom to then reach the bridge again and claim the Strawberry.
Berry 5
Found in the sixth room of the subchapter, this berry is near a circling sludge and dash crystal at the top middle. Stand on top of the lower middle platform and wait until the circling sludge passes the Strawberry, then jump and dash straight up into it, then do another dash diagonally up-right into the dash crystal. Dash at the proper time to get past the moving sludge on the right and again to make it to safety.
Berry 6
Go left of the eighth room of the subchapter to find another room which contains this Strawberry at the left side. For this you'll want to start by standing pretty close to the room transition on the right. When the moving sludges reach the right, do a jump and then a diagonally up-left dash and hold left to reach the dash crystal. Then, quickly a diagonal up-left dash and a straight up dash just after. After the straight up dash, start holding left again and fall into the other dash crystal and do a straight up dash when the moving sludge above passes, then a diagonally up-left dash to arrive at the left area. From here you can collect the berry and get back by dashing into the spring above and doing two dashes back to the right in the air.
Berry 7
At the end of the eighth room of the subchapter, a smashable wall on the floor can be seen which leads to a hidden room containing this Strawberry. The path to collect this berry looks far more complicated then it actually is. Walk off the ledge and do a dash straight to the right, then a dash straight up to get between the dust bunnies. Hold right from there into the dash crystal and dash right and straight up again to get past the next part. Continue holding right to collect the dash crystal and the Strawberry, then dash straight up twice into the spring to bounce you to the left, where you can do some dashes to the left to get back to the start. However, there's also a shortcut to a later room located just above the spring in a hidden wall, so after hitting the spring you could dash right twice to get to this area and continue onward.
Berry 8
In the tenth room of the subchapter, there's a small gap seen in the ceiling which can be entered. The room entered from there will have a Winged Strawberry boxed in the bottom-right corner with no feasible way to touch it. Above it is also a heap of dust bunnies. This berry will need to be collected as it's flying away. Jump to the spring at the left side and wait until you can pass the moving sludge and hit the next spring to be bounced to the right. Grab the wall you arrive at and wait until the other moving sludge is moving down to jump into the spring above and grab the platform the moving sludge is guarding. Quickly jump over it to the next wall, and again to the next wall on the left which you'll need to dash up into in order to hit the spring. After hitting the spring, you need to quickly dash over to the open area in the top-right to collect the Winged Strawberry before it flies away.