I don't think light can be ruled out just because of one word. Light can be measured and is a form of measurement, so it must consist of something. Scientifically it is both a wave and a particle, just because it's not on the periodic table doesn't mean it's not a substance.
As has been already stated by Repo, light is a obvious answer to the first sentence. You need light to help you build - how can you make something if you can't see what you're doing. In a destructive capacity, light bleaches the colour out of objects, it dries the moisture out of everything and can even burn objects if left in light for long enough. A castle needs night to cool the stone or else the mortar would expand and crack from the constant heat.
And if all else fails, everyone knows that 42 is the answer to life, the universe and everything!