Kristiansand is strategically located on the Skagerrak, and until the opening of the Kiel Canal between the North Sea and the Baltic was very important militarily and geopolitically. This meant that for centuries it served as a military stronghold, first as Harald Fairhair's royal residence, then as a Danish-Norwegian fortress, and later as a garrison town. Kristiansand is a gateway to and from the continent, with ferry service to Denmark and a terminus of the railway line along the southern edge of South Norway.

Geologically, this part of Agder is part of the Swedo-Norwegian Base Mountain Shield, the southwestern section of the Baltic Shield, and consists of two main geological formations of Proterozoic rocks that were formed in the Gothic and later Swedo-Norwegian orogenies, with significant metamorphism during the latter. There is a substrate of 1,600-1,450 million-year-old slate, quartzite, marble and amphibolite with some hornblende gneiss, and overlaid on this acidic surface structures of both granite and granodiorite (in general 1,250-1,000 million years old, in some places 1,550-1,480 million years old). The Bamblefelt geological area starts to the east of the municipality and extends to Grenland.

The last Swedo-Norwegian formations are evident in large formations of granite. There are also incidences of gabbro and diorite, less commonly eclogite. The Caledonian orogeny did not affect this area. Faults run southwest-northeast. In ancient times there was a volcano off Flekkeroy, which left deposits of volcanic rock just north of central Kristiansand, on the site of the estate of E.g., now occupied by the Hospital of Southern Norway.