Although most people recommended the use of either 3rd party tools or creative mode, if all you are going to do is a stone castle, then you can do a rig similar to this one:
(height of wall)
XX XX * - blocks
XXXXX X - ground
This is a cross-section of a rig that enables "fast and effortless"1 construction of pure stone walls.
Essentially, you place a magma source (from a bucket) in a high stand (height of your wall), surrounded on the side you don't want magma to flow by non-flammable material. The magma will flow 4 blocks sideways and afterwards will fall down. As soon as the magma curtain is formed, you can then pour water on the sides of the wall. As water flows faster than magma, the wall will be formed as soon as water reaches the ground.
You can then reuse the magma and water. This single rig can make walls with about 9 blocks width. You can combine more to make bigger walls. This is very useful if building in height as you don't need to process or place the stone blocks. This can't do the castle's floors, however.
1 - If you consider fast and effortless picking up a bucket of lava and another for water and then building a scaffold, which you would also need to build regardless.
2 - I've done this in head only, relying on physics that might have changed since the last time I played the game.