If you mean Reave Keep, you only need *rubs temple* 150+ Military strength (and 500 crowns in the treasury), or you can infiltrate it and bring it down using a spy. If you go there, the game will proceed along its plotline but afterwards you get to control when to actually attempt the takeover. The game will let you know if you'll be able to or not.
If on the other hand, you meant Rastedel, then it's not just about your army--you'll need another army to join with Bloodmeen's.
I don't want to spoil things for you. Gaining this Horde is a pretty big deal and there are many ways to accomplish that; and it's actually pretty fun to do. Again, this orc encampment currently has the only unaffiliated army so far in the game and is found near the upper, second portal to this region. It's west (left) of this second portal--just follow the road west from the portal. Specifically, from this second portal it's one road hex north, three road hexes west, and orc village hex. Easy-peasy.
You'll be fine. Just play the game; you're not going to blow it. Have fun getting that Orc Horde Army to follow you! Just remember, you'll need several weeks to get them in line; so take that into account.
I meant Rastedel. But yeah I am guessing you mean the one with Delane in it? If so then already got them to join me. I just got to week 60 and just had the event that fires, it wasn't what I thought it was when I read your post. I thought the week 60 event was the battle for Rastedel itself but apparently it was a counter attack battle.