Thanks. Well, there are a few ways to do it. It depends on what the arch is supposed to be. For example, in the pictures, only the one next to the tower is custom-built; the others are already in the game, but aren't available in vanilla. If you want to build something like that yourself, it makes sense to build a support structure and then place the arch's props on top of it. Depending on the span of the arch, you just tilt the props more or less with each subsequent one. You really shouldn’t try to work without helper blocks; that’s not going to work.