What is the job of a castle priest?

Answer 1

There are no castle priests anymore. I am assuming you mean a castle in medieval Europe, it doesn't matter, a priest's job has always been to administer the sacraments, to celebrate Mass, to hear confessions, to baptize, to witness marriages, to anoint the sick, etc. It has been that way since Our Blessed Lord appointed the first twelve as Bishops, and it will that way until the end of the world. A castle priest just performed his priestly duties in a castle, that's all.

Answer 2

It is unlikely that a priest would spend any time in a castle, since a priest was specifically responsible for a town or country parish and a church.

In some castles there was a chapel and the knight responsible for holding the castle might pay for a chaplain to staff the chapel. A chaplain was a special type of priest who relied entirely on a nobleman for his living - he received money and all meals from the nobleman and in return was employed to recite prayers on behalf of the nobleman and his family (including the nobleman's dead ancestors).

The chapel was generally private and (unlike a priest) the chaplain was normally not empowered to conduct weddings, burials or baptisms, unless given special permission by a bishop.

At Dover castle in Kent, there are two chapels: one in the entrance tower and another, larger chapel inside the donjontower. It is thought that the smaller chapel was for the use of important visitors, while the larger chapel served the knight holding the castle for the king.