Designated an "Urban Heritage Park," Castlefield is an excellent place to begin exploring Manchester, and a walk among the lovingly restored Victorian houses along the old canals or through the reconstructed Roman Fort is time well spent.
Be sure to explore the Bridgewater Canal, constructed in 1761 to transport coal from the mines at Worsley to Manchester, and the many old warehouses that have been restored and turned into offices, shops, hotels, and restaurants. (A trip on one of the Bridgewater tour boats is highly recommended.)