At the end of ROTS, we see Vader standing on the bridge of a Star Destroyer looking at the framework of the first Death Star. It took the entire clone wars to build it that far. Twenty years later the Death Star is finished. Why did it take so long to build? It seems to me that it should have only taken a few more years. Why did the Rebel Alliance completely ignore the building of this Death Star for twenty years? How could the Empire keep it a complete secret for that long?