Yeah, but if I didn't say it someone else was, or they'd just scream unbalanced.
Really, the concept sounds alot like walls, but just for the modern era, but by the time you have this, most of your cities should have a higher cultural bonus than walls/trenches can give, and if memory serves me right, culture or material bonus is used, not both. It is a good idea, but by the time you have the ability to build these, they are more or less obsolete unless you find yourself building settlers and other buildings into the modern era. Not to down the idea, as I said, I think it's a good idea as it just brings walls into the modern era, but by the time you have them, culture is better than any wall.