The Welsh Poems are a series of tales told in verse about King Arthur. The original sources are unknown. Together, the Welsh Poems form a collection of stories about the exploits of King Arthur and Merlin, but they do not form a coherent or linear narrative about Arthur’s life and reign.
“One of the earliest Welsh poems to feature Arthur is Preiddeu Annwn (The Spoils of Annwn). Preiddeu Annwn (sometimes Annwfn) may date to the tenth century.” It describes the voyage by Arthur and his army to the Castle of the Faeries (Caer Siddi) to obtain a mysterious cauldron (Echard).
“Another early Welsh poem with Arthurian references is Pa Gwr, a dialogue between Arthur and a gatekeeper, in which Arthur names his companions and some of his feats” (Echard).
Many of the Welsh Poems have been put into collections, but this does not indicate that they were authored by the same person or were intended to be read together.
The Welsh Poems are mainly important as source material for later authors.
Echard, Sian. The Arthur of the Welsh. http://faculty.arts.ubc.ca/sechard/344welsh.htm