In the beginning of the game you are free to explore the Mountains, the Lake, and the Crystal Forest. The Barren Hills is only unlocked after the other three areas are completed. You need to collect at least four out of the eight Modules in an area and kill its boss to complete it.
The most common order to explore the major areas is the Lake, then the Mountains, and then the Crystal Forest. This order is based on how difficult the area is, how difficult the boss is, and what abilities you have when completing each area.
The final area in the world, located underneath Central Town and only available when sixteen Modules have been collected, is the Abyss.
Detailed World Maps
You can determine the location of a secret by a distinct marking on the ground, similar to those seen on Monoliths, before one. Some secrets require you to press and hold the interact button to reveal invisible platforms in order to access them, some lead to a switch that can be activated with with the use of a gun.
- Hyper Light Drifter Maps 3.0 - on Imgur