My whole upon some ruined tower

Puzzle

My first a useful barrier is
My next from harm to keep;
My whole upon some ruined tower
Does through the night watch sleep.

Hint

Answer

References

• Loyd, Sam [1914]. in Loyd, Sam, Jr.: Sam Loyd's Cyclopedia of 5000 Puzzles Tricks and Conundrums (in English). New York: Lamb Publishing company, page 56.