Roots: The English Civil War, obviously. There are references to Moby Dick, and Brutus's murder of Julius Ceasar. Winter's dialogue is pure Treasure Island.
Goofs: Would Ben really not understand that Sal is offering him prostitutes?
Dialogue Disasters: "Oh lor. The law."
Continuity: The Doctor buys oranges for him and his three companions. He later buys a fruitcake and shares it with Jamie. He gets lost in the TARDIS whilst looking for the library. Whilst he and Jamie are being questioned in jail, he adopts a Scandinavian accent and pretends that Jamie has second sight, quickly persuading Jamie to go along with his ruse. The Doctor has always wanted to meet Oliver Cromwell and gets to do so for the first time here. The Doctor inadvertently lets Every Boy's Book of the English Civil Wars fall into the hands of Richard Cromwell; he hypnotizes him into forgetting all about it. The Doctor tells Jamie and Polly that he has heard of various alternate timelines, including one in which a hereditary Puritan Protectorate rules England until the twentieth century [a reference to Gatiss' BBV audio, Republica]. The Doctor implies that the Master taught him hypnosis.
Ben finds trips back into Earth's history more disorientating than monsters and trips to the future. He drinks rum in the World Turn'd Upside Down, and notes that it always makes him rowdy. He later drinks ale in Amsterdam. He gets coshed behind the ear and press ganged into serving on Captain Stanislaus' ship, which is ironically, like Ben's own ship, named Teazer. Ben murders Stanislaus to avenge Sal Winter.
Polly has an old friend named Rosie, with whom she worked in Bond Street: Rosie, the elder of the two, was heavily involved with the Women's Liberation Movement and took Polly under her wing, transforming her from a shy young girl into a swinging young woman. Frances lends her a simple white nightdress whilst she is staying with her in London in 1648. Whilst trying to gain entrance to Hurst Castle, she pretends to be Spufford's niece. She unwittingly helps King Charles to escape from captivity. Christopher Whyte gives Polly a locket to remember him by.
Jamie and the Laird once spent three days stuck out on the moors. Ben and Polly's experience of traveling in the TARDIS sometimes makes him feel inadequate. He has started to consider the TARDIS to be home.
The TARDIS has a pavilion. The tertiary console room is mentioned (Nightshade). A door appears in a corridor, leading the Doctor into a dark room containing alphabet building blocks, a Turkish rug, clockwork cars, tin soldiers, Victorian dolls, a rocking horse, a hurricane lamp, a box of bricks, and a copy of Every Boy's Book of the English Civil Wars. Another, smaller door, appears on the far side of the room, leading back into the corridor leading to the control room; it vanishes when the Doctor has passed through it.
Links: This story takes place between The Macra Terror and The Faceless Ones. There is a reference to The Highlanders. Ben mentions Daleks (The Power of the Daleks) and Cybermen (The Tenth Planet, The Moonbase).
Location: London, Amsterdam, and on board the Teazer, en route from London to Amsterdam, December 1648; and London, 30th January 1969.
The Bottom Line: "There you go, Sal. With style." If you can accept the portrayal of Cromwell as a noble and honourable soul who did no wrong, The Roundheads is a hugely enjoyable, occasionally swashbuckling, yarn with lashings of Gatiss' League of Gentlemen style humour and great use of this particular TARDIS crew. Just take all the historical detail with a huge pinch of salt, relax and enjoy.