Aldersgate Street derives its name from the old City gate which was the north-western outlet of the City, and St. Martin's-le-Grand (formerly Martin's Lane) from the collegiate establishment which occupied the site of the older or eastern portion of the Post Office. In the last century, that portion from the Barbican to the Bars was called "Pick-axe Street." Aldersgate is supposed to have been one of the four gates of Roman London, and was in the line of an ancient British trackway, improved by the Romans into a road called Watling Street, which came from Dover, crossed the Thames by a ferry, passed along where the modern Watling Street is, emerged from the City by Aldersgate, and went onwards towards Verulamium (St. Albans). As to the origin of the name there are various discrepant presumptions. Some assume that it was so called because it was one of the elder, or one of the four original gates; others that it obtained its name from a Saxon—one Aldrich, the builder or re-edifier of it; but the most probable assumption is that it was so denominated from the alder, or elder trees which grew in great profusion in that locality. The wall, after leaving Cripplegate, proceeded westward for a short distance, then turned at a sharp angle to the south, along the present Noble Street, until it came to near where the Castle and Falcon stands, where it again took a south-westerly direction, past St. Botolph's Church and the Greyfriars' Monastery. As represented by Aggas's map, there were four semicircular bastions in the Noble Street portion, looking westward, and two in the line from Noble Street to Greyfriars, besides the gate at the end of St. Martin's Lane, which is there represented as a heterogeneous mass of buildings, fortified, and with two posterns, the centre arch being hidden by a low building standing in front of it. A little to the north-west of Cripplegate stood a watch-tower called the Barbican, on the north side of the street bearing that name. It was erected by the Romans, and was garrisoned by a cohort of soldiers, who had a threefold duty to perform: first to keep an outlook for approaching enemies, secondly to watch for the outbreak of fire in the City, and thirdly to keep a beacon blazing on the top to serve as a guide for travellers by night over the northern fens and moors. Bridgewater House, which was destroyed by fire in 1698, is supposed to have been built on the site, and now Bridgewater Square. Some remains of the old Barbican were to be seen here in the last century.