Build the foundations of a simple keep with one floor, at least two small stone towers, and a gate. Now take all of your Stone Bricks, and start planning the layout of your Castle. These look much fancier than regular Stone blocks, and they also give your buildings a more professional look. Now place these blocks into a Crafting Table in a 2×2 pattern to get a bunch of Stone Bricks. To do this, simply throw the Cobblestone into a furnace to turn it back into regular Stone.
If you want, you can build your Castle out of this material as it is, or you can convert it into some fancier blocks first. So to begin with your first Minecraft castle idea, consider mining a bunch to get your hands on a significant amount of Cobblestone.
