I surround the castle, deep and wide, Keeping foes away with watery pride. What am I?

Washington's Favorite tree but somewhat obscure by history

I have been used for centuries, since the neolithic period. I have many uses, such as making an area fill with light when lit, acting as a lock for paper, and I also remove hair.

There's a famous one in France. There's a famous one in Rome. There's a famous one if by chance, you pass the burger place by your home. What structure am I referring to?

Born of two, yet neither alone, stronger together than either known. I stand in statues, warm and bold, not quite silver, not quite gold.