Standard Captains are armed with longswords/nodachis, and are capable of blocking attacks and dealing decent damage to players not paying attention to them. Sometimes they will Guardbreak, but they will only throw a player if they are close to a ledge.
Shield Captains are armed with shortswords/katanas and a faction specific shield. (Round for Viking, crest for Knight, rectangle for Samurai.) They behave sort of like Warlords where they will go all block stance and retaliate any attack they block with an unblockable stab. Unblockable attacks or Guardbreak combos can defeat them easily.