The top on a traditional square rigged ship, is the platform at the upper end of each mast. The main purpose of the top is to anchor the shrouds of the topmast that extends above it.
A crow's nest is a structure in the upper part of the main mast of a ship or a structure that is used as a lookout point. This position ensured the best view for lookouts to spot approaching hazards, other ships, or land.