The one thing most British schoolchildren are taught about the Romans (in addition to the layout of a villa and the constituent parts of a centurion's uniform) are that the Romans built straight roads, and two of their roads, the Fosse Way and Watling Street, still run across large areas of the Midlands. So this episode plays on a very familiar aspect of Roman life, and gets a great deal of mileage (mileage - hah!) out of its use of the words 'straight' and 'bendy' on the way.
The plot follows Badvok and Functio's efforts to save their favourite brothel in Romford from being torn down to make way for a new (straight) Roman road. The episode also has a lot of fun at the expense of Romford, but I've never been there, so I can't say how deserved or otherwise this is!
Meanwhile Gargamadua, Badvok's betrothed, continues to flirt with Aulus, leading Functio to save Romford by threatening to sacrifice her to the local god Thingy, only for Aulus to discover and tear down the brothel in the process anyway. For some reason, while preparing for a meeting with Gargamadua, Aulus decides to wear what looks distinctly like Caligula's gold bikini get-up from I, Claudius. Quite why he thinks a practical woman like Gargamadua - or, indeed, any woman - will find this attractive is a bit of a mystery.
What makes this episode so memorable, of course, is the combination of lots of sex jokes, a very familiar element of Roman culture being satirised, and the use of a real place that you can see on a map or pass on a (bendy) road. Although the previous two episodes, about the tension between the British and the Romans and about a Boudicca-like tribe of warrior women, had certainly played on familiar aspects of ancient Rome, it's the combination of a specifically Roman source of comedy with the more everyday and familiar that really works here - culminating a an amusing post-credits segment where a British cart and a Roman cart crash on the new road because the Britons are driving on the left and the Romans are driving on the right.