Not to be missed!
After progressing over the rope bridge in the Desert, you will have to open up a gate by first activating a switch one time, and then throwing two projectiles onto yellow dots at the central artifact. All dots must be blue to unveil the relative secret there at. If you activate a dot, all other dots in a cross form will also become active. This means, that you have to throw your projectiles at two specific dots, so that all of them are lit up altogether.
You can reset this artifact's complete activation of dots by using a ground switch nearby. You can also correct gamma setting under options, in order to better distinct the blue lights from the yellow lights.