On the western front, the Union built a formidable force of river ironclads, beginning with several converted riverboats and then contracting engineer James Eads of St. Louis, Missouri to build the City-class ironclads. These excellent ships were built with twin engines and a central paddle wheel, all protected by an armored casement. They had a shallow draft, allowing them to journey up smaller tributaries, and were very well suited for river operations. Eads also produced monitors for use on the rivers, the first two of which differed from the ocean-going monitors in that they contained a paddle wheel ( and ).