Qaila Jangi became infamous during the American backed defeat of the Taliban by the Northern Alliance, when video footage of US and UK soldiers appeared on television sets across the world, firing into the rebelling Taliban prisoners and an American Taliban was taken prisoner - one of the few survivors.
The uprising started right after the death of an American CIA Agent. There is a plaque commemorating his death but nothing about the hundreds of more Afghans that were killed in the hours afterwards.
After the fall of Mazar-e Sharif to Warlord General Dostum and western advisors, truckloads of Taliban prisoners were herded into the fortress, a large facility half an hour outside of town.
The fortress is divided into two sections, both large empty spaces and the Taliban were placed in one that had some vegetation and a few brick and concrete buildings. The walls are sloping inwards from the outside and vertical from the inside, probably a strong reason why the Taliban were not able to scale the walls and escape after capturing weapons, and why the Afghan and western soldiers were able to trap them in their area by sealing the one exit/entry point and controlling the walls.
A local from the surrounding houses or Mazar-e Sharif to fast talk the guards even if he doesn't speak much english is the surest way to get entry (even if you are unsure about it). There is no entrance fee as the fortress is not a tourist attraction (Entrance fees are rare in Afghanistan anyway), it is still a military installation if unused.
Walking the interior (if you get in) and perimeter is easy. There are fields near the main road giving some space between it and the fortress so photography is best from that side if you want to get wide-angle shots.
Talk to the fortress guards or families (male members) nearby. With a good translator you might get tales of the uprising or their opinions on the state of northern Afghanistan.
Afhgan hospitality gives you a good chance of finding a bed for the night in the houses around the fortress if you want to stay the night and not return to Mazar-e Sharif or continue on westward.