I think Lalibela is the greatest - the largest monolithic rock-hewn building in the world. These churches are special and unique because they are all hewn from the same rock below the ground surface.
The churches are in two groups with their roofs at ground level. The churches are connected through a system of tunnels underground. There are 11 total churches, but one, Medhane Alem, is the largest monolithic rock-hewn building in the world. The churches go 40 feet deep and therefore they are relatively invisible to outsiders. The positioning of the churches serves as protection and lets people practice Christianity safely.
The real engineering amazement is that many of the churches have a water supply via wells. The workers were able to find a way to draw water up to where the city is located on the mountainside.