Introductory campaign – “The Fall of the Griffin” [1 campaign – 1 mission].

I'm curious, how can I defeat Torgul when the threat level is "serious"? I have 19 archers, 9 imperial archers, 60-70 guardians, and 13 orcs. Should I keep running back to the castle to recruit more guardians to carry the battle? The difficulty is set to normal.