This is a design for a game that has never quite been finished. The idea is to use the default Drag cursor to drag the dam walls around. Try and make the falling flow of the polygons as slow as you can without actually stopping the flow. You can use the Replicate cursor (the = sign) to make extra walls, and you can use the Pin cursor to remove them. Note that you can flip through different cursor types by using the mouse wheel. To work as a game, Dambuilder would need a clear goal and a mechanism for having advances and setbacks.
Most programmers instead use Dambuilder for a starting point for an adventure game. Typically they turn the gravity off, and arrange the walls in a more maze-like pattern. The player can then be hunting certain food or power-up critters, and avoiding certain rival critters.
We don't attach any gravity to the player in Dambuilder, so you can use Car Control without any confusion.
Dambuilder has an interesting appearance if you use View | 3D OpenGL Graphics. In order to have good speed, you should use View | Solid Background or View | No Background to turn off the background bitmap, which tends to be expensive in 3D. While in this mode, drive around the world, looking things over. You are riding the player. Try using Ctrl+Up/Down to alter your point of view. Even though this is essentially a 2D game, the program enhances the visual interest by making the walls higher than the critters in the 3D mode. While still in 3D mode, use View | Ride the Player to turn off Ride the Player so you can see the whole world from above. Now try zooming in on the world with Ins, and select View | Keep Player in View so the player is always visible.