Both Spire of Nerathul Wraithstones in Doom the Dark Ages are found on hidden landing spots accessible with the dragon, meaning you're not going to stumble across them while playing the critical path of the mission. You need to go exploring, like you did when looking for the Doom The Dark Ages secret layovers.

The Dark Age’s levels are the different weapon skins you can unlock for your arsenal. Given that your gun is on your screen at all times, these are fun cosmetics to give your Doom Slayer a unique vibe as you’re smashing in the heads of any demon unfortunate enough to cross your path.