"Lego" concrete blocks:

http://www.redirock.net/
http://www.eliteprecast.co.uk/

• Build in days, not weeks or months.
• Simply lift them into place, no need for mortar, it isn't needed to hold them together because they'll stand dry. Perhaps a sealing membrane to seal joints.
• The blocks are reusable. The more standardised they are the better. If you don't need the building or wall any more, just lift them into a new configuration.
• Huge thermal mass.
• When the hordes come howling, they're not going to get through 60cm of concrete.