Hello @Midori0117! As one of the artist, you comment goes straight to my heart!
JRevel explained the main process really well! There's also a little trick about the camera angle we used, and thus the placement of the sprites. Camera is orthographic (not perspective), and angled at 45°. All sprites except the floor are angled in the same way, so that everything is facing the cam, but is still lifted from the ground. This helps to create the "depth" feeling.
Guilloteam - Clement