BEWARE! SPOILERTASTIC! I remember watching 'The Secret of Steel City' as a child, it was one of those odd Eastern European imports Aunty Beeb used to show at tea time. But instead of a sappy milkmaid living in the mountains with her grandpa and a load of smelly old goats (or was it her smelly old grandpa and a load of goats?) we got crazed dictators building mammoth superguns to destroy their enemies! Diabolical gas shells that froze their victims to death! Hideously grim cities that were more giant arms factory than habitation! Border guards who didn't flinch at shooting dead escaping workers! A hero who infiltrates the enemy by getting a job designing weapons (and very good ones at that!) for the evil dictator! By 'eck, that was more like it!
Based closely on Jules Verne's 'The Begum's Fortune', 'The Secret of Steel City' (the literal translation of the name of the city in the novel, which was Stahlstadt) took few liberties with the source material. Only the location was changed (in the novel the two rival cities are built in a remote part of North America by the heirs of the titular fortune) to Eastern Europe, and I think the reason for the cities' existence was changed - in the series, they were just there. And the evil dictator was changed from Professor Schultz to Janus. However, the Germanic nature of the baddies was preserved (Verne based Schultz on Krupp).
SPOILER In one bravura scene, the hero is climbing up a steeply angled tunnel about six feet wide, when he is caught by Professor Janus. Still in the tunnel, he asks Janus where on earth he could have concealed the giant cannon - 'You're standing in the barrel' comes the laconic reply.
Review by McTodd from the Internet Movie Database.