Oh, there are various ways to simulate arches and slopes. For example, if I wanna make an arched gateway, I use a combination of normal walls (or fence foundations), sloped roofs (or ramps or stairs), and sloping half-walls, like in these screenshots.
Problem is, it used to look acceptable before the Blood and Sands DLC. Now that we have real arches in the game, it only serves to underscore the glaring absence of them in the architecture that deserves them the most 