The collection consists of chronologically filed papers (1865-1937) followed by chronologically filed speeches (1886-1933). About 70 per cent of the material is made up of incoming correspondence, and there are large gaps in the period before 1890.
Personal and official correspondence from Wisconsin legislators, governors, civil servants, and congressmen; correspondence and documents dealing with Castle's law practices; family letters; correspondence with friends; material from the Republican State Central Committee; and various documents concerned with such things as church and lodge activity all go to make up the collection. Many of the speeches are political in nature.
Most of the papers are routine, but this is not to say that the collection is not an important one. Aside from the social values whose growth is evident against the backdrop of Castle's life, the collection also has value for political historians because it is large enough in scope to present a well-rounded picture of a man who served within the civil service bureaucracy during a long period in Wisconsin history. The collection offers much evidence about what contemporaries in the party rank-and-file thought of patronage, candidates, and various other issues of their day.
Correspondents include Matthew Hale Carpenter, Henry Allen Cooper, James O. Davidson, Nils Haugen, W. D. Hoard, Merlin Hull, Elisha Keyes, Robert M. La Follette, Sr., Irving Lenroot, Oliver Munson, Emanuel Philipp, Jeremiah Rusk, John C. Spooner, and Isaac Stephenson.
