Although I am not a born Brightonian (the Old Bat is), I have lived in Brighton for 45 years. In fact, I have lived in what is now the City of Brighton & Hove for nearly 60 years. Mind you, the former town has been know as Brighton for rather longer than that.
It started out, as far as we can tell, as Bristelmestune. Indeed, that is how it is recorded in the Domesday Book of 1086, although it was also known as Brighthelmstone, especially for 400 years or so between the 14th and 18th centuries. The name is supposed to be of Saxon origin, deriving from Beorthelm + tūn—the homestead of Beorthelm, a common Saxon name associated with villages elsewhere in England. The tūn element is common in Sussex, especially on the coast. That said, there are examples of people living in the area much earlier than in Saxon times.
A Neolithic (New Stone Age) encampment dating from about 3000BC stood on Whitehawk Hill, there was a Bronze Age settlement in what is now Coldean and Hollingbury hill fort (generally known to locals as the Roman camp) dates from about 200 or 300BC. The Romans did come eventually, in the 1st century AD, and built a number of villas. After the Romans left in the early 4th century AD, the Brighton area returned to the control of the native Celts. Anglo-Saxons then invaded in the late 5th century AD, and the region became part of the Kingdom of Sussex, or South Saxons, founded in 477 AD by king Ælle.
The village had been built where there was easy access for boats and in a spot sheltered by the hills to the north - the South Downs. A stream, the Winterbourne, flowed into the sea to provide water. By the time of the survey for the Domesday Book, the population was about 400 and an annual rent of 4,000 herrings was established.
Now, the city is the largest on the south coast of England with a population estimated at 281,000 in 2014, greater than Portsmouth, Southampton or Plymouth.