There aren’t enough Marines stationed there to defend the embassy. It’s an older design and it doesn’t appear to have been upgraded, at least from the exterior. It’s close to the street and there are no barriers to prevent vehicles from driving right up to the side. In short, it’s not a structure that can be defended by a 6 guard detachment.
Jess, the attacked building is only a store-front. The entire Embassy complex occupies half a block on a steep hill. Check on Google Earth, very detailed pics. Embassy is dug in into the hill (think of San-Franscisco type street, where the old building at the top is the entrance to the consular section only. There are anti-terrorist concrete blocks all around the embassy, it is not possible to drive any 4 wheeled vehicles through them.
Methinks that the Marines were inside the facility, not consular section. Protesters could not enter the facility. Btw. U.S. is building new embassy with the access to Cold war era system of underground installations.
Ambassar residence is far away from the Embassy.