In the middle of the 19th. century, New York was the main entry point for thousands of immigrants ( in 1854 more than 200,000 germans, for instance) so in 1855 the authorities opened up the first "Immigration Centre" at the southern tip of Manhatten, Castle Gardens
In 1892 this was moved to Ellis Island.
Sorry, I have to nit pick a little

Castle Garden existed as a processing station for immigrants between 1855 and 1890. Then there was no processing station for almost 2 years , then Ellis Island, then another period of using the Barge Office, then back to Ellis Island
1624-31 July 1855: no receiving station
1 Aug. 1855-18 Apr. 1890: Castle Garden
19 Apr. 1890-31 Dec. 1891: Barge Office
1 Jan. 1892-14 june 1897: Ellis Island
15 June 1897-16 Dec. 1900: Barge Office
17 Dec. 1900-1924: Ellis Island
See
http://www.rootsweb.com/~ote/ships/castle-garden.htm