Crystalus seems to be some sort of crystaline structure, maybe made from ice. It is not clear if it is artificial or a natural formation. It has a mostly vertical orientation, with lots of ledges and floating and moving platforms.
The Council Member in this level is trapped in a long cave, after the stone door in the lower right side of the level. To get to that door, Keen must obtain the blue gem. In order to get it, Keen must first climb all the way to the top, and fall between the two main floating structures, to get the green gem. With that gem, Keen can climb up the right structure, to the top, open the green door, and get the yellow gem. Then, Keen has to get to the upper left side of the level, to open the yellow door and get the red gem. Once he gets it, he has to return to the right area, and climb up to the red door. After that door is the blue gem that grants him passage to the Council Member cave.
Crystalus is populated mainly by many Poison Slugs. There are also a couple Mad Mushrooms, Skypests and Licks. It is one of the two levels (the other being the Chasm of Chills) where Blue Birds can be found.
Crystalus holds a total of 89,100 points, including 45 Shikadi Sodas, 8 Three-Tooth Gums, 82 (!) Shikkers Candy Bars, 23 Jawbreakers, 2 Doughnuts and 3 Ice Cream Cones (4,500 + 1,600 + 41,000 + 23,000 + 4,000 + 15,000 = 89,100). There are nor drops, neither Lifewater Flasks in here.
Crystalus features a strange bug, where in the last part of the level, if you stand on a higher part of the level and pause or bring up the status panel, it will show you the bottom of the level, marked "edge of map", when you release; doing this elsewhere will fix it temporarily.
See also List of Keen 4 levels.