Alex
Arcane
Alex, right. I wasn't being sarcastic. I meant, "Oh, I knew there had to be an actual in-game reason why someone would want to cast this spell. Traps, yeah, that makes sense."
As for attracting that army, by the time you're high enough level to cast this spell, you'd have Summon Nature's Ally 9, which is also a 9th level spell. So if your DM assumes that this 9th level spell would be roughly as powerful, you could get a couple of powerful temporary meat shields out of it.
Oh, ok, sorry. Completely misunderstood you.
About attracting an army, I always assumed that the attraction range was sensory. That is, as long as you can see the object or area (or can locate it using some other sense), you can be attracted. So I guess you could cast it on something big and easily seen. Maybe a castle? Or a huge flag? This isn't a very nice spell, though. The affected creatures will want to own the thing, even if they happen to be good. You could disppel the magic once the creatures are near, though.