There is no mystery, it just took a lot longer and required more skill with the instruments of the time.
Years ago before laser levels were common, I was involved in an experiment with a classic plumb bob and A frame level, to plot a contour for a berm across a hillside...given the potential for cumulative error from so many readings from sequential datum points, it was remarkably accurate to within a few mm of the line set out using a modern single datum point system, in that case a dumpy level.
Given that the Inca and the Roman Aqueduct engineers could deliver water over incredible distances and difficult terrain using such devices, they certainly work...but I bet every one of those engineers was using the latest innovations of the time to help them, and would have used laser levels and GPS if they were available to them!
Alan