This is a set of extremely well built levels! Azure Garden and Sunk Sea are very well designed and decorated. Your improvement over the years is palpable, and it shows in the opening three levels. Even if Farland Plateau is a bit dated at this point, you have a great sense of gameplay flow in SRB2, and amazing skills decorating a scene.
Also I noticed Jello Jungle hidden here too :threat:
Something you should keep in mind, and this is purely a visual thing, is that your level borders (thok barriers) are often too thin and jagged to make sense out of real geometry. When building them, think "How does this look as a 3 dimensional structure" and that should help them not look so strange.
Gameplay wise I frequently noticed some strangely jagged platforms that are hard to platform on, or would catch me in the middle of gameplay. This is just a matter of fleshing out the geometry a bit more.
Gret work, and Welcome to releases!