Berk, Boni, the Thing Upstairs and Rogg were voiced by Willie Rushton, an English cartoonist, satirist, comedian, actor and performer who co-founded the satirical magazine Private Eye.

But that's nothing compared to the horrors that lurk beneath the trap door, for there is always something down there, in the dark, waiting to come out....The following lines of the theme song, thereupon commence: Don't you open that trapdoor, you're a fool if you dare!

Stay away from that trapdoor, 'cause there's something down there...Creepy, crawly, slimy things, that stick onto your skin... Horrid beasts with tentacles, that want to pull you in... Squirmy worms, slugs and snails, that lie there in a goo...

Beneath the castle are a series of dark and mysterious caverns inhabited by all manner of "horrible things", accessible by the eponymous trap door.

Though mostly hostile or mischievous, the monsters emergent from the trap door include the amiable and periodic Rogg, and occasionally others as harmless as he.

Boni dislikes to be moved from his favourite spot—an alcove in the wall near the trap door—and is often shown failing to warn Berk about the various monsters that come from it.

Drutt (voiced by Nick Shipley) is Berk's pet, resembling an oversized spider, who often causes trouble when chasing after worms and other invaders, as by passing the trap door in search thereof.

The Thing Upstairs (voiced by Willie Rushton) is the impatient, cantankerous, demanding and terrifying master of the castle, who rarely leaves his penthouse room, and consequently is never seen.

In most episodes, he orders Berk (in a Cockney accent) to fix things in the castle, prepare meals for him, or sometimes bathe or clean him.

His appearance is never revealed, but grotesque hints are dropped: For a majority of the series, the plot of each episode will revolve around a new monster that emerges from the trapdoor.

Its latest appearance was the final episode of the series "The Big Red Thing", in which it attacks Rogg before exiting the castle.

Upon catching him, Berk inflates his body through a small hole in the top of his head, before releasing him to soar back down the trapdoor.

In the episode "Fester Rancid", Bubo kidnaps Boni and begins repeatedly hitting him with a stick beside a lake, before he is tossed into the water by Berk.

Bubo appears for the last time in the episode "Scunge", where he returns to irritate Berk but is ultimately sent back down the trapdoor by Rogg.

Berk touched it with the point of a sewing needle, causing it to burst like a balloon and reveal that despite its intimidating manner, it was full of nothing but air.

In the UK, The Trap Door was originally aired during the mid-1980s on ITV in the afternoon children's programming line-up, and later went into repeats on Motormouth on Saturday mornings.