The fortress-like compound rising beside the Tigris River here will be the largest of its kind in the world, the size of Vatican City, with the population of a small town, its own defense force, [and] self-contained power and water. The new U.S. Embassy also seems as cloaked in secrecy as the ministate in Rome. We can't talk about it. Security reasons," Roberta Rossi, a spokeswoman at the current embassy, said. The embassy complex - 21 buildings on 104 acres - is taking shape on riverside parkland in the fortified "Green Zone," just east of al-Samoud, a former palace of Saddam Hussein's. The 5,500 Americans and Iraqis working at the embassy, almost half listed as security, are far more numerous than at any other U.S. mission worldwide. They rarely venture out into the "Red Zone," that is, violence-torn Iraq. Large numbers of non-diplomats work at the mission - hundreds of military personnel and dozens of FBI agents, for example. U.S. embassies elsewhere ... typically cover 10 acres. Original cost estimates ranged over $1 billion, but Congress appropriated only $592 million in the emergency Iraq budget adopted last year. Most has gone to a Kuwait builder, First Kuwaiti Trading & Contracting, with the rest awarded to six contractors working on the project's "classified" portion - the actual embassy offices. Higgins declined to identify those builders, citing security reasons, but said five were American companies. The designs aren't publicly available. Security, overseen by U.S. Marines, will be extraordinary: setbacks and perimeter no-go areas that will be especially deep, structures reinforced to 2.5-times the standard, and five high-security entrances, plus an emergency entrance-exit.