The windows in Japanese castles were lower than windows in people's houses nowadays because the average height of people was shorter a long time ago.
この場合、次のような言い方ができますよ。
ーThe windows in Japanese castles were lower than windows in people's houses nowadays because the average height of people was shorter a long time ago.
「日本の城の窓は近年の家の窓より低いところにある。なぜなら昔は人々の平均身長が低かったからです」
windows in castles で「城の窓」
average height of people「人々の平均身長」
ご参考まで！
カナダ 
