In and around Sussex, a 'twitten' or 'twitting' is a narrow path between walls, fences, or hedges, especially on hills. The most common form is simply a narrow alleyway between two buildings, but longer lanes and back alleys might also be referred to as twittens, if they are a bit closed in. While by no means a requirement, twittens are more likely than your average alley to have stairs.
The word was first recorded in writing in 1801, but had probably been in use long before. It is probably from the Low German twiete, meaning ‘alley' or 'lane’, or from twitchel, which has referred to a fork in the road since the times of Old English, and which has been absorbed into the names of many old lanes and roads. Some have also proposed that it is simply a corruption of 'betwixt' or 'between'.