Mudbrick
Most houses in ancient Europe, Asia, and Africa were built out of mudbrick. You take clay from the riverbank and mix it with water and straw, and pour it into wooden molds in the shape of bricks, and let it dry in the sun. When the bricks are dry, you can use them to build houses. This is a very cheap way to build houses, although it doesn't last very long because rain will gradually erode the mudbrick.
Here's a video of a man in Mexico making bricks:
You can make it last longer by protecting the mudbrick with fired clay rooftiles or flat foundation stones.
If you fire the bricks (cook them) in a kiln before you build with them, the bricks will last much longer. When people built important buildings that would need to stand for a long time, they fired the bricks first.
