I like the idea of a natural sanctuary for the Celts, at least for one the Celts tribes (Gauls ?). The trees were divine symbols to them, and a giant oak would have been a kind of Wonder to them. Of course, you don't build an oak, and anyway, an oak takes ages to reach a massive size. But why not trying to design a unique kind of Wonder : instead of building a structure, the builders could shape a ditch around a mound, and at its top, they would sow seeds of (or plant young) oaks and we would have one or several oaks (an oak circle ?) growing (accelerated, of course) and becoming pretty massive (as a big building) and the Wonder's finishes would be a myriad of relic-kind of stuffs and colorful ribbons hanging in the branches, that the builders would put on the trees, like what we can see sometimes on sacred trees throughout the world (we see that in the Pagan culture). It could be completed also with a few statues or small standing stones. I acknowledge the game's economic and constructive logic doesn't allow the planting of trees with the ressources aimed for a Wonder (stone, wood, gold, etc), but such "natural" wonder would be the closest representation we could get to what the Celts really worshipped, and it will surely give a unique sense of wonder in a Celt settlement. Also, if we consider that the biggest structures in the game are sometimes the kind that actually took years to build (temples, hanging gardens, etc), then the growing time of a tree seems almost acceptable. Even more if we imagine a little bit of Celt magic that could have make trees grow faster... The destruction of such wonder would be as possible as for the other wonders : trees burning and falling + collapse of the mound, back to original ground shape. I admit this idea is pretty difficult to put into the game, but well, I share it anyway. Here's an interesting page on a similar kind of ancient sacred site, mixing tree with land shaping. And here's the kind of oak I think of :