Which Crusade? There were quite a few, taking place over a period of hundreds of years and spanning several continents. Narrowing down the one you're interested in finding out about will probably yield the best answers.

Here is an excellent introductory lecture given by Professor John France (University of Swansea) at West Point (at the New York Military Affairs Symposium, 2012). Also cf. his book of the same title: Western Warfare in the Age of the Crusades, 1100-1300 (Ithaca, 1999).