Hello
@Signe and welcome to the forum!
The symbols you are describing are hallmarks or maker's marks. Hallmarks are marks that indicate and verify the purity of a precious metal (gold or silver) case. A maker's mark is the case manufacturer's trademark. Both hallmarks and maker's marks were required and regulated by many countries and each country and casemaker had their own system of marks. As you might imagine, there are dozens and dozens of different hallmarks and maker's marks seen on watch cases, which is why we need a picture of yours.
A crown inside a circle was seen on German-made gold cases from the 1880s and later:
An excellent resource for hallmarks and maker's marks is here:
https://www.vintagewatchstraps.com/casemarks.phpClick to expand...