In the South (mostly Hesse, BW and Franconia) you have many Celtic hillforts. The bigger ones have been partly identified with Ptolemy's places, and many of these identification are still accepted as plausible.
In the lowlands in the north, there are far less, but still some ancient places, mostly at crossings over rivers or through marshes.
For example, the place where the Alster meets the Lower Elbe is good for crossing, and probably has been some kind of settlement since the time of the amber roads. It might be the place called Treva by Ptolemy.
Or look at the place called
Heidenschanze ("Pagans' Earthwork") near Sievern at the Weser Estuary. 2 hectares within the inner wall, 10 hectares within the outer wall, that is not nothing. Existed from c. 50 BCE until 100 CE.
It was probably a small trading port of the Chauci, quite possibly the Fabiranum of Ptolemy.