This splendid Martello Tower, just south of Aldeburgh in Suffolk, is available to rent if you don't mind eight feet thick walls with rather more brick than windows.
It must make for an interesting and different holiday, and the list of domestic conveniences is quite short. Still you wouldn't have to worry about a nuclear war during your week's stay.
These were of course constructed as a defence against Napoleon and his threatened invasion. I can't tell you if they have left a gun on the top deck so you can take pot shots at passing Frenchies..