The marsh area of The Witness’ map is located between the monastery and the castle. It is quite a long set of puzzles that feature gates, platforms, and many underground rooms.
As you approach the area, you will see a gate with a simple puzzle on it. Simply draw the line round the square, in either direction, to open the gate. The idea of this is to introduce you to the style of puzzles coming up throughout the marsh area. You have to draw the line, leaving the exact room for the coloured shapes. It is easier to understand in the next two sets of puzzles.
See the solutions for the the puzzles up to this point below (click to enlarge).