Hello @Jan_Lovrant,
Currently, there is no way to save parts of Cesium tilesets. This is a feature we are looking to add at some point, but it’s not available yet. You can visit this forum thread if you want to see previous discussion on the topic: How to load maps offline - #3 by Shehzan_Mohammed
You may also find this tutorial helpful - Building Global Scenes with Georeferenced Sublevels – Cesium It demonstrates how to build maps in specific areas.
Thanks for reaching out,
Alex