One way up Snowdon is going up the Watkins path. This brings you from Nant Gwynant (the valley) up via the waterfall. On that morning the valley was still quite foggy because it was freezing over night - this was early January. If you want to go up via Y Lliwedd you need to cross this slate slab bridge or for the straight way up to Snowdon just follow the path up the waterfall.