A Funny Thing Happened on the Way to the Forum, NT Olivier, London<br/>House of Desires, Swan, Stratford<br/>Seven Doors, Minerva, Chichester

Naughtius, Comicus, Gloriosus!

No wonder the Romans tumbled from their imperial pedestal. They were obviously way too busy back home, getting into preposterous sexual pickles. Inspired by the comedies of Plautus, A Funny Thing Happened on the Way to the Forum is, of course, a merrily ludicrous romp with your leery old patriarch, your naive youth, and your bombastic Miles Gloriosus all chasing after the one dumb blonde slave-girl while the brothel-next-door is wantonly confused with a respectable matron's family mansion. It's a mercifully unsentimental musical, with its jovial book by Burt Shevelove and Larry Gelbart and its snazzy jazz and pastiches of music-hall and marching songs by Stephen Sondheim.

No wonder the Romans tumbled from their imperial pedestal. They were obviously way too busy back home, getting into preposterous sexual pickles. Inspired by the comedies of Plautus, A Funny Thing Happened on the Way to the Forum is, of course, a merrily ludicrous romp with your leery old patriarch, your naive youth, and your bombastic Miles Gloriosus all chasing after the one dumb blonde slave-girl while the brothel-next-door is wantonly confused with a respectable matron's family mansion. It's a mercifully unsentimental musical, with its jovial book by Burt Shevelove and Larry Gelbart and its snazzy jazz and pastiches of music-hall and marching songs by Stephen Sondheim.

However, the real delight is that Edward Hall's revival of this 1962 hit proves surprisingly fresh and charming. It must be said the show gets off to a slow start. The chorus-line boys are fit acrobats but feeble clowns, and Desmond Barrit, playing the canny slave Pseudolus, looked unhappy on press night as he joined their opening number, "Comedy Tonight". Perhaps, while facially resembling Frankie Howerd from the film version, Barrit is just too deadpan behind the eyes. The female courtesans' dirty-dancing routines also go on and on, as do their ever-splayed legs.

However, cavils aside, this production is technically swish and becomes ebulliently playful, encouraged by designer Julian Crouch's cardboard cut-out Rome - a collage of magnified guidebook photos plus revolving front doors. Barrit rapidly perks up with his accomplice Hysterium, played by the bug-eyed clown, Hamish McColl (from comedy duo The Right Size). The latter ad libs wittily and is adorable when cross-dressed as a decoy virgin - madly prancing and beaming with joy about being pursued.

Barrit can't stop smiling either during their suggestive duet, "Everybody Ought to Have a Maid", where they launch into a wonderfully lackadaisical, low-kicking can-can. Other treats include David Schneider as the pimp Lycus, dashing about like a gothic villain crossed with a bucktoothed goat, and Philip Quast is having a mock-operatic blast as Miles Gloriosus. Embodying the slave-girl Philia, Caroline Sheen's impression of pea-brained sex-appeal is satirically spot on, singing her own praises with a faintly robotic smile, and Sam Kelly's panting Senex is remarkably endearing. Naughty and nicely done.

The RSC's Spanish Golden Age season is back on lively form as well, with a sweetheart-swapping romantic comedy by a 17th-century Mexican nun. Sister, or Sor, Juana Inés De La Cruz came from left of field. This child prodigy became a teenage favourite in the vice-regents' court in the 1660s, then holed up in a convent with a personal library and carried on writing acclaimed plays. Later, under attack from a censorious bishop, she fiercely defended intellectual women's rights but was apparently overwhelmed by remorse thereafter, renouncing her worldly ways before dying of the plague, aged 44.

She had certainly been in a more mischievous humour circa 1683, penning House of Desires for a festive court celebration of a new archbishop. Her fictional lovers - whom we watch rushing round an aristocratic house looking for their partners in confusion, in the dark and incognito - wryly refer to nunneries as last resorts. Sor Juana also inserted a teasing self-portrait, the romantic heroine Leonor introducing herself as a learned beauty struggling with fame. Director Nancy Meckler underlines the autobiographical element, having actress Rebecca Johnson observe the scene, with quill in hand, before slipping out of her wimple and into the action as Leonor.

This play is, essentially, a rather enchanting bit of fun. There are some longueurs and you're almost bound to lose the plot, it waxes so baroque. Yet the pleasing twist is, the characters admit in asides that they've not got the foggiest either. Sor Juana was, in fact, spoofing Calderón de la Barca's cloak-and-dagger dramas and - though some of the satire is undoubtedly lost on us - the male actors send up hispanic machismo with brio, pouting and flouncing, stamping their heels and constantly yanking out their little blades.

Meckler might have intensified a few more moments of farcical craziness and contrasting poignancy. However, her freeze-frame asides are neat, and she animates Leonor's longest speeches with ingenious simplicity, symbolically exploring the power of words and imagination. As Johnson describes her gorgeous beau Don Carlos (Joseph Millson), for instance, he materialises before the eyes of her addressee and secret rival, Claire Cox's Ana, who reaches out and touches his face as the other woman yearningly pictures it.

This is a visually exquisite production throughout, with a softly glittering gold set and with both flamboyance and finesse in its costuming - grey silk bodices, scarlet petticoats and studded breeches. Johnson sparkles with her own bright-eyed intensity and Millson is surely destined for big-screen stardom. Simon Trinder is also a joy as his cheeky servant, having an absolute ball when he dons petticoats - growing wildly besotted with himself and stealing accessories from the audience. Yes, it's been a week of irresistibly silly blokes in frocks.

Seven Doors, unfortunately, got on my wick. The avant-garde German playwright Botho Strauss is big on the Continent but rarely produced here. So, Chichester Festival is adventurous in programming this radically fragmented vision of contemporary hell - 11 vignettes featuring drab lives with surreal quirks and outbreaks of insanity. In a vast anonymous office, a dissatisfied tenant and corporate chairman end up hissing like serpents. A small-time security guard tries to hire a bodyguard who laughs in his face then sees a monster on the CCTV monitors. A sagging couple are suddenly joined on their sofa by a criminal who rants about Job, burdens of guilt, ferry accidents and addictive repentance. A high-powered academic commits suicide and finds "The Void" is a stupid, ordinary bloke with whom he has to converse forever, etc.

Sometimes, the power games are intriguing and the weird twists are dream-like. Jeremy Sams' translation unsettlingly splices biblical archaisms with colloquial chat. Martin Duncan's production is absorbingly slick too, with an entertaining Stephen Ventura playing the ratty-going-on-raving chairman and Steven Beard as the nervous yet menacingly pushy security guard.

Ultimately though, Strauss's humour can't conceal his pretentiousness. His elusiveness is largely nonsense dressed up as enigma.

'A Funny Thing Happened on the Way to the Forum': NT Olivier, London SE1 (020 7452 3000), to 2 November; 'House of Desires': Swan, Stratford (0870 609 1110), to 1 October; 'Seven Doors': Minerva, Chichester (01243 781312), to 25 September

k.bassett@independent.co.uk