To clear an area, you need to find and activate a portal, resulting in clearing it to close it. Below is a full walkthrough with hints, tips and a few cheese moves that we have found in our video below, The Second portal is found at time stamp 10:33. It is worth noting that each 4 levels, have one portal within them.
Commentary is provided and picks up / drops off, where needed so you can watch the full video without a constant voice over. Good luck with this as it is a really challenging game, it is another Xitilon game that requires a full playthrough (not your average Xitilon).
There are several areas that have constant check points in the form of stone pillars.
There is one missable achievement that is covered at 16:30 into the video.
Enjoy and good luck.
Here is a video guide from the 4lonewolves collective achievement hunters.