The castle is the area where your buildings are placed.
There are 8 expansions in the game. Each one costs always more and need always more time. You unlock the first expansion in the tutorial. The last one costs 2,000,000 Gold and takes 2 days to build. In the expansions you can place all your buildings. For each expansion you buy you will have available 4 new spaces where place buildings. If you want to know something more about castle expansions click here
In the expansions you can place each kind of Buildings (from fountain to training field), but there are 2 buildings that can't be removed, the Arena and the Fusion master. If you want to know something more about the buildings cilck here