 Harrison designed a variety of other structures , one of the most important of which was the replacement of Northgate in Chester , at the suggestion of Earl Grosvenor , mayor of the city in 1807 . The other three gateways , Eastgate , Bridgegate , and Watergate , which carried the footway of the City Walls over the medieval entrances to the city , had been replaced during the 18th century . The earl wanted the structure to be designed in Gothic style , but Harrison advised that it would be more fitting to the adjacent Roman walls for it to be Neoclassical , and after much debate this was accepted . The gateway consists of three arches , a wide central arch for vehicles , and two narrower arches for pedestrians , incorporating pairs of monolithic Doric columns . Harrison then designed Denbighshire Infirmary in North Wales , built between 1810 and 1813 , and in about 1820 he made some internal alterations to the Chester Exchange . In 1820 Harrison was consulted by Magdalen College , Oxford , about improvements to New Building , a block of the college built in 1733 , and on other alterations . His plans for New Building were adopted , and the additions were made in 1824 at a cost of £ 2 @,@ 600 ( equivalent to £ 210 @,@ 000 in 2015 ) . 
