I discovered that building a castle this way is surprisingly fun and easy. On a traditional castle, you have to carefully plan out dimensions. With a "building block" castle, you can just start tacking on 4x4 rooms somewhat randomly, and it looks cool! Inside, instead of a large courtyard, you will have long, potentially winding passages, but that kind of makes it fun.
The only thing you need to plan are staircases. The staircase will fit entirely in a 4x4 block (as shown below), but you need a landing room at the top and bottom, so this necessitates that you have three blocks in a row with the staircase in the middle. Other than that, your passageways can snake off in any directions you want.
Once you have a staircase, you can just build 4x4 rooms exactly above those on the lower floor. The openings between rooms can be the same as the previous floor or totally different. You can have one long passage or have lots of T-junctions and dead ends. Maybe you can add a door to a dead end to make a special room. Just start tacking on 4x4 blocks and see what happens!
The 4x4 blocks work well for corridors and little rooms at dead ends, but I found that you can do a bit more by combining four 4x4 blocks together into an 8x8 room (6x6 interior area). Below are two examples - a bedroom and an entertainment room. The big screen TV was inspired by this Pinterest pin and was achieved by dropping the floor of the room by half a block (using cobblestone slabs for the floor instead of whole blocks).
Yes, you can make 4x4 modular castles in Builder Buddies, too:
Be creative, and have fun!!!