The Wall is Hadrian's, built to defend Roman Britain against the unpacifiable raiders to the north, but now fallen into disrepair. The sourcebook contains information relevant to British knights visiting Pictland, and for creating Pictish characters.
Pictland, or Caledonia, is tribal, with inroads of feudalism and Christianity. Picts are matriarchal, and practice Heathenism, with its own set of virtues. Each clan has a totem spirit, which can bestow particular gifts on warriors, but demands they follow a geas. Warriors may befriend other spirits, obtain their tattoos, and call on their magical gifts. I misunderstood how this is supposed to work the first time I ran a game, but it's really not that complicated, and fits well into Pendragon 's trait system. There are also rules for shamans, who use the new Spiral Walking talent to treat with the spirits.
Pictish culture is brought to life by an open-ended campaign for Pictish characters, three short adventures suitable for Picts or Britons, and a long adventure for British knights on a mission to pacify the Picts.
There are maps and a chapter describing the forts, cairns, and magical places of Caledonia -- unlike some Pendragon supplements, the dscriptions reference the map! Unfortunately, there's no index or glossary -- the latter would be useful since there's extensive use of Pictish terms. The character sheet indicates Christian virtues, rather than Heathen.
Beyond the Wall is a worthwhile extension to a Pendragon campaign, or an interesting place to start a new campaign. Fans of Glorantha may also find Beyond the Wall useful, since many of the Pictish customs work well for the primitive Hsunchen.