watchtower

As a noun beacon is

a signal fire to notify of the approach of an enemy, or to give any notice, commonly of warning.

As a verb beacon is

to act as a beacon.

As a noun observatory

is a place where stars, planets and other celestial bodies are observed, usually through a telescope.

As a noun taxonomy

is the science or the technique used to make a classification.

As nouns the difference between watchtower and lighthouse

is that watchtower is an observation tower, in which a lookout keeps watch over prisoners, or looks out for fires etc while lighthouse is a building, usually a tower, containing a light to warn or guide ships.