Yes. In Scotland a civil marriage ceremony can be conducted by a registrar or an assistant registrar in any licenced location, including Culcreuch Castle. This is similar, for example, to being married by a justice of the peace in the United States.
A religious wedding ceremony may be conducted at any suitable location by a minister, clergyman, pastor, priest or other person entitled to do so under the marriage (Scotland) act 1977. The Castle holds a list of local ministers for all the main religious faiths, christian and non-christian.