Llanberis is on the way from Caernarfon to Snowdonia Peak on road A4086. In Llanberis you can park your car near the railway station (there is a huge, but expensive parking – £12 – gps 53.116407, -4.118919). There are other car parks where you don’t have to pay for a whole day and are much cheaper. I think the one just before the station is much cheaper (gps 53.115212, -4.119055) and isn’t displayed on Google maps.
From the big parking: After parking your car, walk to the shops but go left and then take the first street right. Here you see a small sign “Ceunant Mawr waterfall”.
A little further you have to cross the river and railway to a path that follows the river at the west side. After 5 minutes you will have your first view at the Ceunant Mawr/Llanberis falls. The walk, along the river, to the waterfall is nice but not very special. The Llanberis falls are nice to see, especially after rainfall. July 2013 (when we visited Llanberis falls after the train ride) there was quite a lot of water in the river and the waterfall looked lovely. At the right side there was another single drop.
The river Afon Arddu plunges down over 30-60 meter and is pleasant to see but the Llanberis falls itself (the part you see from the viewpoint) is approximately 20 meters high. There are several other drops in the river upstream but these are difficult to view.
When taking the train to Snowdonia mountain in Llanberis you can see a second waterfall if you will sit at the left side of the train! Here you will have the best views if taking the Snowdonia Mountain railway. Tickets aren’t cheap but it a nice train ride. Unfortunately for us there was so many wind that the Snowdonia train had to stop halfway the top. We waited for 30 minutes for the wind that maybe was lying down, but we had to go back. We got a refund, but it wasn’t what I had expected… If you want more information like ticket prices and a schedule of the Snowdonia Mountain railway visit the official site.