A more thorough explanation:

Definition: An open town is a city in a war zone that is not defended and is vulnerable to being taken over by attacking forces.

Example: During World War II, the city of Paris was declared an open town by the French government. This meant that the city was not defended by military forces and was left open to the German army. The Germans were able to take over the city without much resistance.

Explanation: An open town is a city that is not protected by military forces and is therefore vulnerable to being taken over by enemy forces. In the example of Paris, the French government decided not to defend the city in order to avoid unnecessary destruction and loss of life. However, this decision allowed the German army to easily take over the city.