Kilty Party by Anna Markland

Secret tunnels, benign ghosts, and ruined castles—the ingredients of a mystery. Right? Except Kilty Party is a romantic comedy. Shaw Drummond and Caitlin Blair balk at King William’s command they marry to end an age-old feud between the two neighboring clans.
As luck would have it, they unexpectedly fall in love at first sight, but other family members seem determined to thwart their union—particularly Shaw’s bossy sister and Caitlin’s older brother. However, defying a king comes with consequences and royal dragoons eventually arrive to confiscate both castles in the name of the crown.
When Caitlin mysteriously disappears without a trace, accusations of shenanigans fly from all sides.