Londinium was established shortly after the invasion of Emperor Claudius in the summer of 43AD. The first Roman building was an Auxiliary Fort, constructed on the north bank of the Thames to the east of the Walbrook on the instructions of the military governor Aulus Plautius. A civilian settlement comprised of native tradesmen and foreign merchants developed outside the defences of the fort, which was itself demolished after only a short period of use. Londinium centred on Cornhill and the River Walbrook, and it expanded west to Ludgate Hill and east to Tower Hill. Suburbs existed at St-Martin-in-the-Fields, Westminster, and around the southern end of the Thames in Southwark.