I can remember watching this story very young actually, and it's one of the few that I can watch today and remember seeing back then (the rest seem to have gotten lost in a fog of memory). Besides that, it's one of my favourites because it's a great story during one of the best times in Doctor Who...Tom Baker as the Doctor and Louise Jameson as Leela, his companion.
Just the fact that the entire story takes place in a lighthouse was a stroke of brilliance. It's a perfect setting for this horror-laced story, brimming with tension and suspense a-plenty. The entire cast gives great performances and there are some classic Tom Baker moments: The survivors or a ruined ship take refuge and start to bicker amongst one another, and the Doctor is just sitting there amidst it, until he suddenly interrupts: "Just a moment! We haven't been introduced!" and then slumps back in his chair. Additionally, later on in the story, he returns to the survivors and says, in his usual maniacal fashion, "Gentlemen, this lighthouse is under attack and by morning we might all be dead. Now, who's interested?" Classic stuff, to be sure. Not to mention we finally see the nemeses of the Sontarans, the Rutans.
I would heavily advise this story to anybody looking for a primer in not only the Tom Baker era of Doctor Who, but Doctor Who in general. I've shown this as an introduction to several friends who were unfamiliar with the show and they've become fans as a result, so I suppose I must have chosen wisely! LOL