The prime difficulty is that for the bulk of the story, we follow "the lighter people," the non-noble characters, as they play three successive tricks on one another: first a steward, Malvolio, finds a forged letter that deceives him into behaving pompously before the countess Olivia, whom he now thinks is in love with him; then the delightful idiot, Sir Andrew Aguecheek, is tricked into provoking a near-duel with Count Orsino's young manservant "Cesario" (who is really the disguised girl, Viola); finally Malvolio is hied off to a dungeon and succored and visited as a madman by the rest of the lighter folk, ostensibly because of the behavior encouraged in him by the forged letter.
All this takes up quite a bit of time, and it all taxes the modern reader's understanding because the non-nobles, not only Andrew and Malvolio but Sir Toby Belch, Maria (she forges the letter), and Feste the clown, speak in language much more obscure than that used by their betters. They speak, I venture to guess, like common people of 1600. Their banter is full of lost references to popular songs and jokes, they are forever joshing each other in that pun-infested, can-you-top-this style that Shakespeare loved (Samuel Johnson complained about his "quibbles"), and naturally they use the slang and idiom of the time, some of it bastardized French, some of it simply vocabulary long disused. A "catch" is a song, "Catay" is China. Decipher this, either on the page or in the theater, and by the way I know people who attended a performance of Twelfth Night just this past weekend.
TOBY: "My lady's a Cataian, we are politicians, Malvolio's a Peg-a-Ramsey, and [sings] 'Three merry men are we.' Am not I consanguineous? Am I not of her blood? Tilly-vally, lady. [sings] 'There dwelt a man in Babylon, lady, lady." (II, iii, 69-72)
There's plenty more, and why does it always seem that the helpful notes at the bottom of a page of Shakespeare never elucidate what is truly bizarre? Your scholarly editor will assure us that "skill-less" means "without knowledge," but will not explain why Viola wants a beard, but "would not have it grow on my chin" (III, i, 45). Good luck to all.
So oddly enough it is the daunting-looking columns of poetry given to Viola, to the Countess Olivia, to Sebastian and to Duke Orsino which seem to swing aside like doors and, actually comprehensible, open up the movement of the play beyond. As an example of this sheer mental refreshment, here is Viola. She has been sent in her male disguise from her master, Orsino, to woo the mourning and cloistered Olivia by proxy. "What would you?" Olivia asks, amused and attracted by the beardless "Cesario." Viola answers. She would
Make me a willow cabin at your gate
And call upon my soul within the house;
Write loyal cantons of contemned love
And sing them loud even in the dead of night;
Hallo your name to the reverberate hills
And make the babbling gossip of the air
Cry out 'Olivia'! ....
At least we understand this as English, or think we do. It sounds very feeling. But we may yet be deceived. This may not be as dreamy and deeply meaningful a speech as changed modern ears are apt to hear. Explanatory notes at the bottom of the page, useful at long last, tell us that the willow is a symbol of grief for unrequited love, so the image of a shack built of willows at someone's door might have been intended as simply enjoyably silly. The same for "calling upon my soul within the house" -- imagine some lovelorn person wandering around literally saying "Upon my soul," as characters did in plays and books until that oath fell out of use. Because we're not used to glowing language (and who ever gets used to Shakespeare in any case?) we may see profundity, and make extracts, where the great man intended pleasant absurdity.
While the reader is deciphering the lighter people's three successive barroom tricks, the nobility carries on, behind its swinging doors of poetry, with the love story. Its focus is the entrancing disguised girl, Viola. She has been shipwrecked along with her brother, Sebastian, on the coast of Illyria. Separated by the storm, each thinks the other is drowned. Viola dresses as a man, and takes service with the Duke, Orsino; her chief duty is to pursue his romance with the Countess. Olivia rejects him, but falls in love with the new page, "Cesario." For her part, Cesario falls in love with the duke herself.
It remains only for Sebastian to enter the fray, exactly resembling his sister, to astonish everyone and prompt questions, accusations, and confessions of truth and love, and thus launch the action softly down to its denouement, all the while he eyes Olivia himself. Did Shakespeare mean for us to draw any profound lessons from it all? I doubt it; only the fine expression "Some are born great, some achieve greatness, some have greatness thrust upon them," thrice repeated, suggests a peg upon which a grand theme might hang. And yet the fine expression comes from Maria's forged letter, so possibly it is all in fun, too.
The introduction to Twelfth Night in my 1960s-era volume of Shakespeare says that this is one of the three "Joyous Comedies," the other two being Much Ado About Nothing and As You Like It. Very well, but four hundred years is a long time for one civilization's definition of a joyous comedy to endure. It's a long time specifically for an upper class, courtly definition of joyous comedy to endure -- the joy of the gentleman's belly laugh at the idea of a steward being persuaded a gentlewoman loves him. The joke on Malvolio is about far more than his yellow stockings. I wonder what went on at the theater this weekend; I wonder how any theater company can make a modern audience understand and relish this play. I'm not such a virtuous and democratic killjoy that I can't laugh at Malvolio, but the entire world that bred him and his mockers (and they included everyone, beginning with his fellow servants) has gone, and how are we to resurrect them, even in make believe? For us pale twenty-first century weaklings, it's far easier to understand Viola and Olivia, arguing like soft-voiced birds over personal identity. "I prithee tell me what thou think'st of me"/"That you do think you are not what you are," etc.
OLIVIA: Love sought is good, but given unsought is better.
VIOLA: By innocence I swear, and by my youth,
I have one heart, one bosom, and one truth,
And that no woman has; nor never none
Shall mistress be of it, save I alone. ... (III,i,153-157)
Beautiful. The writer of my '60s era introduction also dwells much on the love story, but I think Shakespeare liked his servants and his Belches better here. And just how do we go about properly appreciating that?