noun
observation tower; lookout tower
A tower or elevated structure built for viewing scenery, often found in parks, tourist spots, or harbors. The gloss 'conning tower' is a specialized naval term for an armored pilot house on a warship; in general use, 'observation tower' is more common.
港の展望塔から夕日を見た。
I watched the sunset from the harbor's observation tower.
展望台 is an observation deck or platform, often part of a building or tower, while 展望塔 specifically refers to the tower structure itself.
Compound of 展望 (てんぼう, 'view, outlook') and 塔 (とう, 'tower').