Why is the job of a historian difficult when studying this time period

Historians study the past by interpreting evidence. The historian works by examining primary sources -- texts, artifacts, and other materials from the time period. ... The interpretative writings of historians --books, journal articles, encyclopedia entries -- are considered secondary sources.

The main goal that Queen Isabella and King Ferdinand was striving for was to conquer land in France, make the pope recognize their authority, end feudalism throughout their territory, and make all of Spain Christian.