This level consists of 'layers', areas with their own themes, tileset, music and boss(es), each with their own sets of enemies and challenges.


Most mandatory layers have four quadrants:

- The NW quadrant is where you enter the layer from.

- The SW quadrant leads back to the Lift, a column of screens which can take you up to any previous layer, or to the Surface, an area with infinite health, infinite weapon energy and a shop.

- The NE quadrant leads to the boss of that layer. You need all the (blue) boss keys of the four main layers (layers that are 2 screens tall) to finish the level by acquiring a certain weapon in the lowest layer, guarded by key doors.

- The SE quadrant leads to the next layer.


Purple keys are obtained by completing the challenge within the purple portal on each mandatory layer. These keys are used to access the Lift in the SW quadrant of all layers except the first and last mandatory layers, which are free to enter.
Purple keys can also be used in the shop on the Surface to unlock Pile Driver and Magnet Missile, weapons that can open weapon energy caches in some layers as well as dig through the floor in other layers, skipping a major part of the next one.


Notes:

- Entering the purple portal in the ice layer takes you to the start of the NE quadrant of that layer. (Make sure to defeat the boss in that quadrant before taking the portal, unless you wanna do that whole quadrant again.)

- There are four secret bosses throughout the level, hidden within the orange portals in some of the layers. (The first secret boss is on the Surface: go right, past the shop...)
-- The last secret boss is on the lowest layer and requires all the weapons of the previous secret bosses to access. (Go to the right door first when you enter that secret layer, I should've put key block checkers on the left door)
-- You can use all the secret boss keys together to complete the level, with the main difference between this route and the normal route being the bosses you face and the weapons you unlock, but both paths have a weapon that heals you and a spammable weapon.

- After defeating the final boss and getting the weapon required to access the Energy Element in the shop, you can go up or down using the Lift, instead of only up. in the Lift, you can now access the extra, optional boss fights there, rewarding E-Tanks, M-Tanks and a Life for each fight completed.