My interpretation of it is that each of the character, the fireboy, the watergirl and the castle represent a piece of humanity

The boy obviously represent action, passion and extroversion. The girl represent thoughts, self, and introversion. The castle represent harsh reality, social expectation, and judgement.

When the girl first arrives, her first thought is to inspect herself and get to know who she is. After knowing content on the knowledge she is happy.

Then the boy come, his first thought was about his environment, who was a threat? He also inspects himself but in a more active way by throwing himself around

The two being the caught sight of each other. the boy, being more extrovert, try to initiate first, but both realize they couldn't interact with each other in a safe way. however they still decide to be together

The castle being the judgement of the outer world intervene. the boy bravely stands his ground, the girl knew better and fled. Seeing a sign of weakness the castle attacks the fleeing girl and crushed her. Overcome with grief and sadness, the boy's passion turns him into a self destructive fireball that crashed violently into the castle.

The destroyed castle created a bare bone skeleton that can move. this means that without fire(passion) and water(thoughts) it's just a soulless machine walking around doing the same until it breaks down.

The fire leaps up and wrap itself around the skeleton creating muscle, now the machine has strengths and passion, however, it's inner side is bare to the whole world, it's thoughts and opinion come out freely and perhaps even violently without hesitation to the consequence.

Now the rain falls down and soaks the body in skin, cooling down the passion and focusing the mind. the machine can now think about it's own protection. with all three combine humanity is complete.