There are 100 challenges. This guide is going to demonstrate the way you can complete the Fortbyte.
Fortbyte Challenge #50: Accessible at Night Time Inside Mountain Top Castle Ruins — Fortnite (Season 9).
The ruined castle on the top of the mountain can be found alongside Haunted Hills. At the top portion of those ruins you may find Fortbyte 50. Bear in mind, it must be nighttime in your game!
Mountain Top Castle Ruins – Byte Location