Arguably Bulgaria's ugliest town, this place does have its charms, especially if you are interested in Communist-era industrial history.
With its huge heavy industry factories, some of which in various stages of dilapidation as they were abandoned years ago when planned economy collapsed, it is something of a Bulgarian Manchester or Pittsburgh. It is worth trying to explore the enormous halls, now overgrown with grass and even trees, from the inside, but most visitors are content with just passing through by the main road, which is in fact the best observation platform. Drive slowly.
Where in Bulgaria are you?