The calculated distance (air line) between Dortmund and Castrop-Rauxel is approximately 7 miles respectively 12 kilometers.
By car or train, the actual journey to Castrop-Rauxel is certainly longer, as only the direct route (as the crow flies) between Dortmund and Castrop-Rauxel has been calculated here.
Air line (approximately)
12 kilometers
6 nautical miles