The Red Zone is the high-security area in the centre of Islamabad containing the Parliament, the Supreme Court, the President's House, the Prime Minister's House and most foreign embassies. The whole area is sealed off by police checkpoints and protected by concrete blast barriers — the legacy of the 2008 attacks that culminated in the truck-bombing of the Marriott Hotel.
Ordinary visitors cannot drive through but can be dropped at specific points (the Faisal Mosque is just outside the western edge; the Convention Centre is at the eastern edge). The pedestrian access through Aabpara and Constitution Avenue gets you closest to the government buildings.