ah.. then,

2. Naut. A barrel or cylindrical box fixed to the mast-head of an arctic, whaling or other ship, as a shelter for the look-out man.
1818 Blackw. Mag. IV. 343 The Crows-Nest is..generally a cask, fixed near the mast-head, to protect the observer from cold, and enable him to look out for whales, or open pieces of water.

but,
1. Mil. ? A fort placed on a height. Obs.
1604 E. GRIMSTONE Hist. Siege Ostend 163, 2 Frenchmen..fled unto the enemies to the crowes neast.