There are many forts in the US.
Remember up until after WWII we had active coastal artillery batteries that are fairly sizable.
In my Arctic Raiders Campaign, I had the players training at one such place that had been turned into a recreation area and park. But when the war broke out, it was militarized, and they moved troops into the underground galleries.
We also have a good number of Civil War Era Forts, some earlier forts mostly on the eastcoast.
An example of two on the west coast are, The Presidio and Alcatraz both build as early Spanish Forts.
Here are some links to some of the forts we hace localy:
http://www.cdsg.org/HDCRdata/stevensx.htm
http://www.ftmac.org/index.htm
The above was my playground after dark, especialy the portions that where no longer used and closed
http://www.laokay.com/TheGunBatteriesofSanPedro.htm
And then they also had Nike Zues Missile Silos:
These are all over the Los Angeles area as well as the country.
http://www.nps.gov/goga/nike-missile-site.htm
Further, after BRAC, the Base Realignment and Closure of the 90s and early 2000s, there are a good number of large and small bases and facilities that are dormant, and would be still standing in the 2013 Twilight War.
And then, you also have the ability to turn so many places into forts. A ski lodge that's primary access is via a cable car or a narrow trail up a steep slope. I know of three places within an hour of me where this would be the problem.
Isolated communities on penninsulas and islands would be an easy place to defend.
LARGE Buildings would be good, like a Hospital, or even large building groups that one finds in larger cities, where several tall buildings surround a general courtyard.
those are just some ideas off the top of my head.