It appears that Prince Llewelyn Terrace is still in existence, but could have changed its name. If you take the road out of Penygroes towards Nantlle, there is a turning (Ffordd y Brenin) on the left, which leads up to the school. Behind the houses facing the main road there is a short terrace of houses before reaching the school. This is Prince Llewelyn Terrace. I've no idea if they are the original buildings or if they have been redeveloped.