Formal.
The Ceremony is on grass, and there will be a long night of dancing, so if you can rock heels through that go for it!
Yes, there is an elevator to go up stairs, and there will be transportation onto the grass were the Ceremony is.
They are at the same venue.
The Ceremony will be outside, and the Reception is indoors, with an outdoor firepit area. It could be cold so make sure you are you are prepared for a chilly night.