The defenders are centered around the stronghold. An idol inside on the table is a clue if the spawn is active or not. Also, normally, mounted opposition crowd around the stronghold. Once the spawn is active, you will see much more, various types of opposition. After you kill the boss, there is a cooldown. I'm not sure what it is, but I'll check and report back to you in the morning. And yes, when you enter the region of your opposing guild type, they will get a message.
might i also add there is a quest you have to do first in your respective area to see the gate to go in and ether kill your opposing sides monsters or go defend your side.