Although El Camino de Santiago is a specific route, pilgrims can journey there along many other routes from as far away as Notre Dame in Paris, Lisbon in Portugal, and Cadiz on the southern tip of the Iberian peninsula.
Catedral de Burgos, Espanha torres com campanários
Leaving a stone at the cross at Cruz de Ferro is a long tradition, and the size of the mound of stones is an indicator of the extent to which people follow it (i.e., it's a huge pile). Some leave things other than stones -- pieces of clothing, notes, toy cars, etc. Few seem to know how old the actual iron cross is there, but there are hints in the literature that the tradition is ancient.