Ok, it has a long long distance to I Capture the Castle (here why I love it) but I see that the latter was an inspiration. You can tell "Keeping the Castle" is a lighter/simpler version. Nonetheless I like it. 
 Althea was witty and headstrong. Mr. Fredericks was also headstrong and he didn't much care about manners. A clash had to occur. This is a funny love story with lovely characters and enjoyable atmosphere. I had some doubts that it could have taken place in Regency era (perhaps a half century later) but still, I really have a nice time. 
 The best summary gave QNPoohBear in the second paragraph. 
 I have one warning. Don't expect to much of this book. Because if you do you will end up with feeling like Emily May had. Just have a fun.