Malory places Castle Dangerous within the story of Gareth, the young man initially known at Arthur's court as Beaumains. Lady Lynette seeks a champion for her sister Dame Lionesse, whose castle has been besieged for nearly two years. The besieger is Ironside, the Red Knight of the Red Lands, not one of the other coloured knights defeated earlier on the road. Lionesse remains inside her own stronghold while Ironside dominates the approaches. Describing her as his prisoner can obscure this distinction: the danger is an enforced siege, not his ownership of the castle.

Gareth approaches a field of tents and a grim warning. Nearly forty defeated knights hang from trees with their armour and identifying equipment still upon them. An elephant-bone horn hangs from a sycamore, ready for another challenger to sound it. Lynette explains that Ironside's strength grows during the morning until he has the strength of seven men at noon, then diminishes. She advises Gareth to delay until the knight's advantage has passed. He refuses to arrange the challenge around that weakness and blows the horn, bringing Ironside out to meet him.

The encounter becomes a prolonged struggle on horseback and then on foot. Both men lose equipment and suffer wounds as their swords batter shields and armour. The fighting extends through noon and towards evening, interrupted by a rest. Lionesse watches from the castle, and her presence gives Gareth fresh determination. When Ironside knocks away his weapon and drives him down, Lynette's cry that her sister is watching helps him recover. Gareth regains his sword, overwhelms his opponent and is finally in a position to kill him. His victory releases the castle from the immediate threat.

Ironside asks for mercy, but Gareth confronts him with the shameful deaths of the earlier challengers. The Red Knight explains an oath associated with a lady who wanted vengeance upon Lancelot or Gawain. That explanation does not undo the killings. Other nobles plead for his life, and Gareth imposes obligations: he must submit to Lionesse, make restitution for damage to her lands, appear before King Arthur and seek forgiveness from Lancelot and Gawain. Mercy therefore takes the form of changed allegiance and accountability, rather than treating the hanging of defeated men as acceptable knightly behaviour.

The castle also belongs to the later stages of Gareth's recognition and courtship. Malory uses Castle Perilous as well as Castle Dangerous, so the alternative name is not a later invention by Tennyson. The tournament and subsequent marriage arrangements should not be collapsed into a single wedding feast at the besieged gate. Tennyson's Gareth and Lynette reworks the material through a more openly allegorical sequence of opponents. Across the retellings, the fortress remains the destination that reveals Gareth's identity and ability. He arrives as an already knighted rescuer, not a kitchen servant who becomes a knight only after winning this last fight.