You might wonder why anyone would bother watching a decades-old educational show, and there are really two reasons. The most obvious is nostalgia: though it was made and broadcast years before I (and, I assume most of my readership) was born, it was still being watched on tape at british primary schools well into the mid-90s, at the very least, along with most of the other Look and Read series that had been going since the early 1970s. Luckily for any adult viewers, some helpful soul by the name of malvolio80 has uploaded the series to Youtube with all but one of the awful educational sections mercifully excised. From what I remember, those parts were so slow and simplistic to be useless even when my age was in single figures, and I can only imagine how tedious and annoying they'd be to an adult viewer.
The other reason is the aesthetic: a dusty old castle full of books and ghosts, mystery-solving teenagers in shabby hand-me-down jumpers and master criminals that drive around in a knackered old van. The makers can't have known at the time, but Dark Towers is almost perfect as a simple introduction to some of the basic concepts of hauntology and late 20th century british folk-horror. Obviously, it's not only a childrens' show, but one made to be shown in schools, so the horror elements aren't particularly strong, and the budget is very low, but the look, the feel and the general ambience of the series work well.
You won't really be missing out on a great deal if you don't watch Dark Towers, but I assume it'll at least have some nostalgic value for most British readers, and it's not a terrible show either (in its edited form, at least). For anyone else interested in British kids' horror, something that skews a bit older in its target audience, like The Owl Service, Chocky, or Elidor, might be a better choice.