All pioneers in a row
Pioneers are specialists who can capture an area without the need for towers.
Pioneers are well suited to capture areas that towers cannot easily reach, or for areas where a tower would not be worth it. In mountains, they can help reach valuable deposits such as gold long before they could be reached with a tower.
Pioneers can move enemy border stones, if they are not protected by a tower. Unlike in Settlers III, however, they will refuse to move allied borders.