To unlock and access the B-side level for Chapter 7: The Summit in Celeste, follow these detailed steps to find the cassette tape and then reach the hidden entrance.

Locating the B-side Cassette Tape:

1. Start in the Main Level: Progress through the chapter until you arrive at the room with two large windows.

2. Find the Access Way: At the top of this room, there exists a small passage situated between the two windows.

3. Jump to the Access Way: From the larger platform positioned centrally in the room, execute a jump and a dash maneuver to successfully reach this small access way at the top.

4. Navigate the Platform Room: Inside the passage, you will encounter a room filled with blue and pink platforms that alternate in time with the background music. Be attentive to the rhythm and carefully time your jumps to navigate this room without falling.

5. Collect the Cassette Tape: Successfully maneuver through the alternating platforms to retrieve the cassette tape. This cassette will unlock the B-side level.

Reaching the B-side Entrance:

1. Continue in the Main Chapter: After collecting the cassette tape, advance further into the chapter. You should aim to get approximately 1500 meters into the level.

2. Jump Over the Ledge: As you progress, look for a ledge to jump over, and then use climbing techniques to ascend to a tall platform.

3. Locate the Hole in the Ceiling: Once at the top of this tall platform, shift your focus to the left. Here, you will discover a hole in the ceiling.

4. Dash Through the Hole: Execute a dash to go through this hole, which will lead you directly to the hidden B-side entrance for Chapter 7.

5. Enter the B-side Level: Once inside, you will now have access to the B-side version of Chapter 7: The Summit, which offers increased difficulty and new challenges compared to the main chapter.

Summary

To summarize, you need to successfully navigate the music-timed platform room near the large windows to find the cassette tape. After collecting it, progress to around 1500 meters into the chapter and look for a hole in the ceiling after climbing to a tall platform to access the B-side entrance. Good luck tackling the more challenging B-side level!