The street layout, typical of the old quarters of many northern European cities, is made especially picturesque in Edinburgh, where the castle perches on top of a rocky crag, the remnants of an extinct volcano, and the main street runs down the crest of a ridge from it. This crag and tail landform was created during the last ice age when receding glaciers scoured across the land pushing soft soil aside but being split by harder crags of volcanic rock. The hilltop crag was the earliest part of the city to develop, becoming fortified and eventually developing into the current Edinburgh Castle. The rest of the city grew slowly down the tail of land from the Castle Rock. This was an easily defended spot with marshland on the south and a man-made loch, the Nor Loch, on the north. Access to the town was restricted by means of various gates (called ports) in the city walls, of which only fragmentary sections remain.