The atmosphere of fear and suspicion in war-time was really well explored, though unfortunately the book was marred for me by the revelation of an almost pantomime villain at the very end. I did enjoy the twists and turns where seemingly friendly characters became untrustworthy, and sinister ones ended up being more heroic than they first appeared. I found this cover very appealing -- note the green-painted lighthouse, another detail I wasn't aware of previously. There is even a touch of the supernatural in Petra's relationship with the standing stones, the Daughters of Stone, and the dangerous sandbar called the Wyrm that haunts her dreams.
Our Castle by the Sea is a beautifully designed book that approaches questions of loyalty and betrayal from a fresh angle; and one spooky element for me was the description of the boat-shaped church that also featured in Storm Ahead.