If you used SignifierOne's tool (or was it someone else?) that made a bigger, more elaborate Rome, then it is possible to create ditches for water. But as the water itself, I'm afraid the engine doesn't support it. You can't even have rivers on the same map as cities. Of course, you can trick it a bit by making your own "water" which is a building with water texture.
It'd be a real challenge though, since extremely few has done completely new city plan (and not just custom buildings).
My advice, judging by that picture, is to make the pyramid and buildings as the plaza of the highest city level. Ignore the waterways and use the eastern buildings as the regular buildings. Put them in a Roman city plan and it should look similar to the picture, minus the waterways and smaller in scale.
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