1
general::
A dependable person on whom one can lean in time of trouble, as in After Dad died Grandma was a tower of strength for the whole family. This expression, first recorded in 1549, originally was used most often to refer to God and heaven, but Shakespeare had it differently in Richard III (5:3): "Besides, the King's name is a tower of strength."
American Heritage Idioms
2
general::
Phrase(s): a tower of strength
Fig. a person who can always be depended on to provide support and encouragement, especially in times of trouble. • Mary was a tower of strength when Jean was in the hospital. She looked after her whole family. • Jack was a tower of strength during the time that his father was unemployed.
McGrawhill's American Idioms And Phrasal Verbs