Sirom's Shield

Sirom's Shield is a massive towering wall that forms the border of the inner-city of Sirom, protecting it with a combination of conventional military engineering and elven protective magics.  This wall also provides the structure for the inner-city's multitiered vertical layout. The outter areas of the city are protected by a more modest Lesser Wall of Sirom that provides some protection to the local farmlands, villages, industry, and highways.  
Originally built by human occupiers when the city was transformed into The Lagos Citadel, the fey folk turned it their advantage when the city was reclaimed (by entering the city through the heavy dark magic subterranean cave systems).  Now, it serves as Sirom's greatest asset, allowing it to maintain its sovereignty despite a much smaller military force.  Construction efforts are being made to expand Sirom's Shield down into the subterranean depths, but it's currently considered low priority due to the heavy dark magic.