One of the comments made during the public consultation about the character statement for the Old Town Conservation Area (OTCA)¹ is that 'twitten' should not be used to describe alleyways in the area as it is a Sussex term that was not used in Brighton. Oh yes, it was.
      Unfortunately, the word has been replaced throughout the OTCA report by 'lane'.

The earliest citation in the OED is dated c1798 and is from Thomas Pennant's A Journey from London to the Isle of Wight. Against the sidehead 'Old Town' he writes: 'The more antient [sic] town stands on the west side of the Steyne, . . . it is of a square form, and consists of several parallel streets, which finish at the south cliff . . . there are no lanes or cross streets, nor have the parallel streets any other communication than alleys, or, as they are called here, twittings, narrow passages often not three feet wide, scarcely pervious to two bulky people, should they chance to meet.'²
      William Harrison Ainsworth, describing the 'Great Escape' of Charles II in his novel Ovingdean Grange (1860), mentions the 'twittens' in the Old Town several times, and Erredge's History of Brighthelmston specifically defines twittens as the narrow lanes between Middle Street and Black Lion Street³.
      Twice in 1852 the Brighton Gazette referred to fires in the twittens⁴. In 1881 the Brighton Herald reported that 'A fire, which might have proved of a disastrous character, considering its locality, broke out in Brighton early on Wednesday morning, a in a range of workshops, wholly constructed of wood, forming a portion of old Brighton, and situate in the narrow "twitten," as the "ancients" still call it, which leads from Ship-street to Middle-street, known as Ship-street-gardens.'⁵

A correspondent to the Brighton Herald in 1877, local bookseller Charles Hindley, remarked that the word was already dying out.

All old residents in Brighton can but have observed how the word 'Twittens" in reference to "the Lanes", situated in the very heart of the town, has been gradually but surely going out of use. We now seldom or never hear of he or she "coming down, being in, or passing through the Twittens."
      Yet, apropos to the word 'Twittens," I may mention, en passant, that no later ago than Friday last I was directed to apply to a person on a certain matter, residing, my informant told me, in Ivy-place (leading off Waterloo-street), then to me a perfect terra incognita. Being near to the spot as directed, I made further enquiries of an elderly gentleman in respect to the place I wanted, when he informed me that if I turned "up yon twitten I should find Ivy-place to the right of me."⁶

The last word goes to a charming but probably apocryphal anecdote by an anonymous contributor to the Derbyshire Advertiser and Journal, writing about Brighton under the heading 'Places worth visiting'.

Those who would form an idea of what Brighton was like in the olden times must visit the "twittens," or lanes, such as are situate between Middle-street and Black Lion-street, for instance, narrow as the famed Calle of Venice, and generally running at high angles with the principal streets. Some of these lanes are so narrow that it is not always easy for two persons to pass each other, and it is related of George the Fourth, who in the zenith of his popularity at Brighton was an early riser, and accustomed to ramble unattended in all parts of the town, that he was once taking an early walk before breakfast when he met in one of the twittens Mrs. A——, a lady no less remarkable for her embonpoint than the Prince himself. To pass each other was impossible, and Mrs. A——, curtseying to the Prince, began to back out. The Prince, however, politely took off his hat, and saying, "Mrs. A——, I cannot allow you to go back," retreated backward, conversing with Mrs. A——, until, arrived at the end of the lane, he courteously stood aside for the lady to pass, and then re-entered the twitten.' ⁷

Etymology
'Twitten' is indeed a Sussex dialect word and is defined in Rev W D Parish's Dictionary of the Sussex Dialect (1875) as 'a narrow path between two walls or hedges⁸. The OED definition is virtually identical.
      The Oxford Dictionaries website suggests that it derives from an Anglo-Saxon word, twicen, meaning a place where two roads meet, and twitchel, a word for a forked road used in the north Midlands as early as the 14th century. Old Danish had a word, tvede, and modern Danish the word tvedelt, meaning forked or bifurcated. It is also said to be related to 'betwixt' and 'between'.
      It is almost certainly cognate with the German word Twiete for a passageway, found particularly near the north-west coast of Schleswig-Holstein and in Hamburg, where there is appropriately a Fischertwiete. This is likely to relate to the German zwischen (pron. tsvishen), meaning 'between'.
      One can reasonably speculate that seafarers crossing the North Sea carried the word between the two coasts.