Hey there!
Another software you might consider is Tinkercad, especially if you're just starting out. It's quite user-friendly and great for simpler models. However, for the level of detail you're aiming for, Blender, as mentioned above, might be the better choice.
One thing to add about creating your cloud base is considering its stability and how it will support the castle when printed. You might want to design a hidden support structure within the cloud that can hold the castle up without compromising the aesthetic. For the fine details, consider creating textures in a program like Substance Painter, as it can add depth and realism to your prints without the need for intricate modeling.
And remember, printing a complex model like this might require supports, so plan your model in parts that can be easily assembled. This way, you can avoid printing the whole thing at once and reduce the risk of print failures. Happy modeling!