Your goal is tu build a bridge that enables the creatures to cross the abyss. You build the bridge in design mode. The bridge tracks placed on the green lines are those where the creatures can walk. Usually you build triangular or trapezoidal brackets to support these horizontal tracks (the triangular brackets are better because they do stronger bridges). You can use wood or metal: the wood is cheaper but it doesn't resist too much weight, so it's good to be used for supporting purposes; the metal is more expensive but you can use it at the horizontal tracks because it resists a lot of weight. Search the combination of materials that allows you to build a bridge before running out of money. When the design is ready you can test the bridge: if it resists the weight of the creatures crossing it you win and you can go to the next level.
In design mode, click on an anchor point to create a new bridge section. To erase a bridge section, click on it, or select an anchor point and press the DEL key. When you finished preparing the bridge in the editor, click at 'Test Level' or press the T key to test the bridge.