The book is written for quite young children and it has plenty of pictures. I don’t find it easy to say which level it would be because it was not written for adult learners. The language is simple, but some of the words and expressions may not be taught in lessons, though they may be familiar to children whose first language is Welsh. I would guess that Sylfaen might be the closest match.
Here is a short sample.
“Roedd y twysog Manawydan yn teimlo’n unig a digalon. Roedd newydd gladdu pen ei frawd Bendigeidfran yn y Gwynfryn yn Llundain, ac yn crwydro strydoedd y ddinas yng nghwmni’i ffrind Pryderi.”
Naturally it gives a shortened version of the stories.
I’m not sure if that helps. I tend to buy a lot of books and many of them are way beyond me at the moment, but I keep them in hope that I will be able to read them some day.