Invasions are held at:

-Ul'dah ---> Camp Horizon
-Gridania---> Camp Tranquil
-Limsa ---> Camp Bloodshore

Its not just in Horizon and its not just in Camp Tranquil Either...

THERE EVERYWHERE!!! EORZEA IS DOOMED I SAY!! >.< Defend our Great Continent!!

* Please note that I did not see anywhere on forums that Camp Bloodshore was under attack so Its something added so that all 3 nations are under attack!*