Mesopotamia is a big word that means the land between two rivers called the Tigris and Euphrates. It's the place where some of the first people lived a very long time ago. They built very cool and fancy buildings that we call architecture.
Their buildings were made out of mud called mud-brick, and they used something called a ziggurat to show how important their buildings were. A ziggurat looks like a big step pyramid made out of mud-brick. It's like a huge set of stairs that lead up to a big temple on the top.
The Mesopotamians also invented something called a arch, which is a curved shape that can hold up heavy things without needing any walls. They used arches in their buildings to make them very strong and so they didn't have to use as much mud-brick.
They also made something called a vault, which is like an arch that goes over a whole room instead of just a doorway. This made their buildings even stronger and allowed them to build bigger and bigger rooms.
Overall, the architecture of Mesopotamia was very advanced for its time and used some really cool techniques that we still use today.