I have almost all the items from the Lego Castle Fantasy set, and I would like to build a nice full Lego diorama, but I have no clue on where to start. Do you know if there is a tutorial on how to plan the diorama and how to build the terrain to make it solid enough to sustain the items and movements?
If you have access to some Duplo elements, these make good "bulking" blocks for larger terrain sections - as the 2x2 and larger bricks fit over the top of the duplo studs you can build a fairly stable base using those, and then work on the sides and top with System elements as needed.
Otherwise it comes down to sensible building techniques such as starting with larger blocks, overlapping them correctly so that each layer provides support to the ones around it.
There's a good answer on how to build a large structure for transport, which talks about using Technic bricks and pins to connect the sections together, as well as the answer on connecting levels of a multi-story building using tiles and jumpers or 1x1 plates to provide some grip while still enabling access inside.