Command rank, command rank, and command rank are the top three
ingredients. I successfully threatened away two consective Woodmen
capitals as the Dog Lord in game 101 - with all of 800 troops. BUT
with artifacts a 120+ commander. As the Noldo I lost a 65 loyalty
MT/castle to Tonn Varthkur in game 85 - same thing, he had less than
1000 troops but a high command rank. Nothing else seems to correlate
with success.
I think it's a little more complicated than that. Loyalty is
definitely a factor, as are dragons. With two or three
dragons in an army it's possible to threaten just about
anything, no matter what the size of your army. I've
threatened a city/castle with 400 men and a 60-odd
commander + 3 dragons.
I'm attempting to bang together a rudimentary formula, with limited
success so far.
As far as I can work out, the formula someone posted a day
or so ago is about right, though probably with an overal
tweak to make threatening more difficult.
Colin.