A gate or entrance — the structure that serves as an opening to a yard, building, or enclosure.
学校の門で友達を待ち合わせた。
I met my friend at the school gate.
城の大きな門がゆっくり開いた。
The castle's large gate slowly opened.
夜になると庭の門を閉める。
We close the garden gate at night.
A school or branch of learning based on the teachings of a single master — a sect or group of disciples (often used in expressions like 'to knock at someone's gate' meaning to become a pupil).
彼は有名な教師の門を叩き、弟子入りした。
He sought out the famous teacher and became an apprentice.
その流派の門には多くの門下生がいる。
That school has many disciples.
In biology, a 'phylum' — a major taxonomic division within a kingdom (e.g. Chordata).
脊索動物門には人間も含まれる。
Humans are included in the phylum Chordata.
教科書では動物門ごとの特徴が説明されている。
The textbook explains the characteristics of each animal phylum.
A counter for cannons/guns — used to count the number of guns (in military contexts).
その戦艦には二十門の大砲が搭載されていた。
The battleship was equipped with twenty cannons.