Radar Artillery and Civil Defense buildings are both brilliant additions to the
Yilar Entrenchment Scheme (The Y.E.S.

)
According to my combat calculator, a Mech Infantry (12.18.2) would win, even against Modern Armor, 78% of the time in a city with Civil Defense.
Obviously, this would be even better in a Metro with Civil Defense.
EDIT: Bah - I forgot that a Modern Armor is a 3 move unit... that means they'd be able to retreat from combat with our Mech Infantry.

So, basically - we're looking at 32% win, 31% retreat, and 36% lose to the Modern Armor.
An attacking Marine would only have a 4.7% chance of killing a Mech Infantry in a city with Civil Defense.
This would vastly reduce the number of Mech Infantry we need to hold against even a very determined assault.
Of course, we'll need to maintain significant offensive forces/bombers in case they decide to land a stack of their own Mech Infantry and combat settlers.
Too bad most of this goes out the window when FREE/Saber get nukes.
I can't remember - in C3C does a nuke strike wipe out all the defenders? or just redline them?