Hello!
For a project like this, I'd recommend starting with Blender. It's free and has a lot of features that can help you with both the castle and the cloud. For the castle, you might want to use the sculpting tools to carve out the intricate details. Blender's sculpting tools are very versatile and can handle the complexity of your design. For the cloud, the particle system could be used to simulate fluffiness. You can also use volumetric shaders to give it that misty, magical look.
Don't forget to keep an eye on the model's complexity since too many details can make 3D printing challenging, especially on a smaller scale. It might help to separate the model into parts for printing and then assemble it later. Good luck!