We at DigitalArsenal have been working on a live Cesium code editor as a tool for some of our customers.
We decided to host it as an online demo at https://mapshot.app.
It has many of the same features as Sandcastle, enabling users to share code through dynamically generated links.
It also allows the user to download a single-file, serverless version of their work as a .html file.
We integrated Microsoft’s Monaco Editor, the UI behind Visual Studio Code. The editor also uses Cesium’s new TypeScript declaration file, which lets the user access the full Cesium documentation live in the editor, in the same way as Visual Studio Code (see below).
It is free to use, feedback welcome at email@example.com