tell me something that exists only in your mind and nowhere else

I circular turreted castle with a draw bridge, flowers, a waterfall well in the centre that's in the middle of a great forest. The well goes down into a stream that leads to the ocean and dolphins sometimes visit. It started off as just a well in a forest when I was a kid but it grew over the years. I like it there, I dream about it as well as imagine it to feel peaceful.