Yea, there actually are a lot of those pop-up missile turrets around DC. The problem is, there is so much air traffic flying right down the Potomac River to Reagan National airport, that having a plane suddenly veer off course, go over the Lincoln Memorial and start straffing the White House, that I'm not sure that they would be able to do anything before it was too late. So that's a lot more believable than having a bunch of foot soldiers with big rifles and RPG launchers get near the White House. They've had all the roads for two blocks around the White House closed since 9/11 and there are multiple rings of security set up purposely to stop that kind of scenario where a few heavily armed guys rush the White House.