Here is the full list of all 10 Hexton achievements.
Place 100 buildings in a classic map
Place 250 buildings in a classic map
Place 100 buildings in a plains map
Place 250 buildings in a plains map
Place 100 buildings in an island map
Place 250 buildings in an island map
Place 100 buildings in a valley
Place 250 buildings in a valley
Place 100 buildings in a donut map
Place 250 buildings in a donut map