The youngest son, now the king, had an underground castle built, in which he imprisoned his daughter, and then killed the architect.
The youngest son decreed that whoever could find his daughter would marry her, but whoever tried and failed would die.
The king borrowed the sheep and took it to his daughter in the underground castle, using a magical charm to open the doors.
When the three days were up, the shepherd returned to the king to retrieve his sheep and the young man revealed himself.
This method of winning the princess is also found in the Italian fairy tales The Fair Fiorita and The Golden Lion.