The countryside of the British Isles is dotted with hill forts, earthworks usually on top of hills that were used in the first millennium B.C. by Bronze and Iron Age Britons. While some of the hill forts are tourist attractions, most of them are tucked away on mountainsides, in agricultural fields or other out-of-the-way places. Now, an impressive new online atlas reveals the location of more than 4,000 of them, reports Steven Morris at The Guardian.

Dozens of experts and volunteers compiled the atlas over the course of five years, Morris writes. Gary Lock, emeritus professor of archaeology at Oxford, tells Horatio Clare at The Financial Times that the project is an attempt to get the public to recognize and protect the historic structures that are sprinkled around them.

According to the BBC, around 40 percent of the 4,147 sites in the atlas are in Scotland. In Ireland, the counties of Mayo and Cork top the list with more than 70 forts each. There are 1,224 forts in England including many in Devon and Cornwall. For some reason, a few areas, like Kent, have almost no forts.

Hill forts don't exactly live up to their name, Clare reports. Many are not built on hills and many do not resemble forts. And, in fact, archaeologists no longer believe they were actually military forts or defensive structures at all. Instead, these earthworks were likely places of gathering for people to feast and trade.

While many of the forts are on private land, there are quite a few accessible to public. The atlas team hopes people will begin to plan walks to visit the forts and make efforts to preserve them.

Though most hill forts are in out of the way places and don’t need much looking after, some are facing pressure from human development. For instance, in 2015 archeologists put up a cry when developers unveiled plans to build homes close to Old Oswestry Hill Fort in Shropshire, which legend says is the birthplace of Queen Guinevere of the King Arthur legend. Those plans were approved, though the homes have not yet been built.