Okay, so imagine you have a big house and you want to divide it into different rooms. But you don't want people to see or hear what's going on in the other rooms. So you build walls to separate them.
Now, a chemise wall is a special kind of wall that was used in old castles and fortresses. It is a very thick and strong wall made out of stones or bricks. The wall has two parts, an outer part and an inner part, with a space in between them called a "moat." The moat was usually filled with water or dirt to make it even harder for enemies to get through.
The purpose of a chemise wall was to protect the castle from attackers. If the outer wall was breached, defenders could retreat to the inner wall and continue fighting from there. The moat in between the two walls also made it difficult for attackers to climb up and over the walls.
Overall, a chemise wall was a very important part of the defense of a castle or fortress. It helped keep people safe and protected during times of war or conflict.