1A 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.
2A 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.
3In 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.
4A 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.