Carlos did not speak Castilian when he arrived to Castilla, even it is not showed in the series.
Network and producers planned to continue this series with one centered around Philip II, which together with Isabel (2011), The Broken Crown (2016) and this series would have chronicled the history of Spain (and to an extent Western Europe) from mid 15th Century to the end of 16th Century. However, ratings were not as strong as expected, and the Network put the project on hold.
In order to have the complete series DVD box set available by Christmas season, it was put on sale before the last four episodes of the show had aired.