You need to keep the first two blocks (solid and empty). Also, you can't generate a castle from just one block, you need to split it into many blocks that fit next to each other. Then you need to set connectors for your blocks. You can read how they work here. In your screenshot, the block has a 0 connector on top and a 1 connector on the bottom. It will only place another block above it that has a 0 connector on its bottom face. But there is no such block in your example.