I just won my first one completely on my own vs one of the smaller adventuring companies. The battles vs the large spawn armies are tough. I think I lost 12 guys taking my first castle but you need a critical mass of decent troops because while only 12 died it took 2 waves and about 40 were wounded. Much of that large # is low level troops which die fast. The sieges vs castles in PoP are the same sizes as vs towns in vanilla so if you've taken town before it is similar what you need to win most of the time. I didn't take my first castle until had 90 men and was about level 23.
