It's a very regular shape and will be difficult to both hide (from residents in your game worlds) and show (to viewers in the real world via your map). It could be the result of a planned layout by the city founders...out in the open as you find in Washington DC and similar cities. Anyone that knows magic, and looks at a map, however, will realize what's going on.

Have you seen Iron Man? Tony Stark's blueprints for his whatsit were drawn on several sheets, containing other drawings - and you could only see the final result by layering them and holding them up to the light. You might take that idea, with part of the shape contained in the construction of the sewers, part in the roads, part in the buildings themselves. Individually you can't see what the symbol is but, taken as a whole, the thing still works like a magic circle. Then again, it'd be hard to show that on a map I think.
M