Hi,
I have no idea about Hildesheim, but a lot of German cities which look old, are pretty new. One example is the old town of Dresden, another is Münster. The amount of accuracy varies quite a lot. In Münster, they moved they widened the streets to accomodate cars.

However I would recommend Berlin for your visit. It is quite close (around 2h by train). If you interested in strange medival restaurations/reconstructions you should visit the Nicolaiviertel. It is a pseudo-medival city center constructed out of concrete slabs ("Plattenbau")