<<Disclaimer: Verify this information before applying it to your situation.>>
 
Yes you can use granulated sugar instead of castor sugar.  Castor sugar
is the same as granulated sugar except that it is finer.  In Europe
Castor sugar is used in baking instead of granulated sugar becuase the
granulated sugar is coarser.  But in North America granulated sugar is
finer so there is really no need for castor sugar.
hope this is clear
Michelle