Boyd’s Bible Dictionary:
(warning). In Hebrew sense, a place manned, provisioned, and fortified (1 Sam. 13:23; 2 Sam. 23:14; 1 Chron. 11:16); guards (2 Chron. 17:2; 1 Chron. 18:13).
Concise Bible Dictionary:
A place strengthened temporarily for war, or permanently for the protection of the country. The same name is applied to the soldiers who guarded such places (1 Sam. 13:3-4,23; 1 Sam. 14:1-15; 1 Chron. 11:16; 1 Chron. 18:13; 2 Chron. 17:2; 2 Cor. 11:32).
Nimrod Castle – Northern Israel – 13th Century