blockhouse (n.)

"detached fort blocking a landing, mountain pass, etc., 1510s, of uncertain origin; perhaps from Middle Dutch blokhuis, German Blockhaus, French blockhaus (from one of the German words), all from 16c.; see block (v.1). Later "building with an overhanging upper story with loopholes for firing through" (often a square of logs serving as a fort in rough country), which seems to connect to block (n.1). For second element, see house (n.).