Very rich citizens, ones who can afford a fully trained war-horse are form competent militia - a huge asset, and despite their lack of military experience.
The riches available through Novgorod trade allow for an exceptionally well equipped, no worse than any noble army, militia of merchants . Expensive horses, excellent arms and armor, allow them to fight on equal terms with any enemy of their Great Novgorod. Their lack of battle-forged experience makes them less steadfast then professional soldiers, however.