What does the 'black castle' symbol mean?

[thundermonkey]

Check out attached, I can't figure out what this means, I'm sure it is basic b/c practically all my cities have it, barracks maybe?

I apologize for the stupid question, I have been waiting for the mac version to come out, it was only released a short time ago, so I am a newbie. I've only had it for a few days.

 

[acidsatyr]

its defense bonus for units in your city

 

[Esox]

It's your city defense, and the following number, +85%, means that units in the city will have +85% strength when defending.

EDIT: beat me to it