The assembly site was called the "Mittelwerk" or "Mittelbau" - this is sometimes said to refer to its general location in the "middle" of Germany, and sometimes said to refer to the fact that the V2s were assembled in the middle part of this tunnel system. It was a tunnel system in an old gypsum mine in the Kohnstein hill near Nordhausen, on the southern border of the Harz Mountains. Two parallel tunnels were dug through the mountain, linked by a ladder-like series of lateral chambers (that is, it looks a lot like a curved ladder in plans). The site was first planned for strategic petroleum reserve storage, but was appropriated by the SS for construction of V2 and V1 rockets (and other projects), after Peenemunde was bombed.
The adjacent concentration camp, to house slave laborers from Buchenwald, was called "Dora" (sometimes also called "Mittelbau"). At first, the slaver laborers were housed right in the tunnels - exposed to constant noise, dust, and cold humid air, with only the most primitive of living facilities. Later the camp was built.
The Soviets blew up the tunnel entrances after the war (after the US, British, and Soviets removed most of the V2 assemblies), and the tunnels were buried. One of the main tunnels was reopened by the Dora Memorial Site as a museum in 1995. Guided tours are given to the public in this part of the tunnel and one of the side chambers.
When they explored the reopened tunnels, including some flooded parts, they found an amazing array of V1 and V2 parts that had been left by the Soviets (the floors in the sections just beyond the public part are still littered with V1 parts today). Most of these are still there, although some V2 engine assemblies have been removed and sent to various museums.