Here's a thought: limbo is raw unconscious where anything is possible, any creation a reality if you will it. And while we don't know how long exactly Saito was trapped down there, it's clearly decades. We saw what Dom and Mal made in a similar amount of time- an entire world. Saito could've done the same. He could have made his own world. He could have made anything.
And yet, he doesn't.
He, very specifically, makes the castle that was the bottom layer of the dream when Cobb was trying to extract his expansion plans.
It seems like an odd choice, doesn't it? Why recreate the place where someone else tried to steal from you and, as far as we can see, nothing else to be your entire world for who even knows how long? It always seemed odd to me until I realized something: while Saito had clearly done his research on Cobb, that second level of that initial dream job was most likely the first time they really met. Saito knew he was going to be stuck in limbo for a long time. He knew that being in limbo meant running the risk of losing his mind and waking up with no memory of who he is. And he knew that, if anyone was going to come after him, it would be Cobb.
So he rebuilt the place he first met Cobb in an attempt to, whatever else he forgot, never forget the man he hoped, the man he knew, would come for him
