We parked near the remains of the city wall (51.704333, 10.000083.) built between 1252 and 1305. Walked along the fortifications, named Wallanlage Tourlaville, in honour of the French twin city. It is a beautiful place, with several ponds and walking paths along them.
There's also this historic structure, a sort of small waterfall.
Then we turned to the very interesting puppet theatre building (51.705250, 10.003889). The theatre group Das Theater der Nacht (Theatre of the Night) was founded in 1988. It was originally a touring theatre, but in the mid-1990s, after discovering the former Northheim fire station, the town got a proposal to convert the building into a puppet theatre. After two years of renovation, the new theatre opened in 2001.
Opposite the theatre is the evangelical church of St. Sixti, founded in 1470.
We walked around the town, there are quite a lot of half-timbered streets, though in the very centre a modern shopping centre has been built.
The market chapel of St Fabian and St Sebastian is one of the oldest religious buildings in Northeim and was built between 1354 and 1391. As you can see it consists of two halves. The nearer, half-timbered, is the 1734 "Old Guard" (Alte Wache), where the garrison guards of the town governors were located.
Getting there: we parked here 51.704333, 10.000083. There is a railway station Northeim (Han).