Hide the symbol within other streets.

Build them as streets and alley ways that cover a nicely rounded hill, and each street or alley way has a convenient gutter that runs down the middle of it. All other street's gutters nicely bypass the shape's gutters/drop into covered sewers.

When it comes time to use the great summoning symbol or whatever, someone at a temple/hidden store house on the top of the hill takes an axe to a huge store of oil barrels, and sends a wave of oil down the gutters, gives it a good ten count (or whatever it would take for the oil to flow through a good chunk of it.) and then drops a torch on it.

Whoosh, magic symbol the size of a city. You can even hide the circle inside others that wrap around the contours of the hill.