Tower Ridge is the longest of the north face's four main ridges, with around 600 metres of ascent. It is not technically demanding (its grade is Difficult), and most pitches can be tackled unroped by competent climbers, but it is committing and very exposed. Castle Ridge (Moderate), the northernmost of the main ridges, is an easier scramble, while Observatory Ridge (Very Difficult), the closest ridge to the summit, is "technically the hardest of the Nevis ridges in summer and winter". Between the Tower and Observatory Ridges are the Tower and Gardyloo Gullies; the latter takes its name from the cry of (French for "watch out for the water") formerly used in Scottish cities as a warning when householders threw their slops (and worse) out of a tenement window into the street. The gully's top wall was the refuse pit for the now-disused summit observatory. The North-east Buttress (Very Difficult) is the southernmost and bulkiest of the four ridges; it is as serious as Observatory Ridge but not as technically demanding, mainly because an "infamous" rock problem, the 'Man-trap', can be avoided on either side.