It is undoubtedly one of the most ancient castles in the kingdom, though it was greatly enlarged in later years, and was kept up until the reign of Elizabeth, when it was abandoned as a stronghold, and allowed to fall into decay.（A Yacht Voyage Round England by W.H.G. Kingston）
英文を前から読んでいくために、語句のかたまりを作ります。こんな具合に課題文をスラッシュ（斜め線）で区切ります（スラッシュリーディング）。語句のかたまりを意識すると、リスニングで英語の意味をつかむのが楽になります。
It is undoubtedly one of the most ancient castles in the kingdom,／（接続詞の前）though it was greatly enlarged in later years,／（接続詞の前）and was kept up until the reign of Elizabeth,／（関係副詞の前）when it was abandoned as a stronghold,／（接続詞の前）and allowed to fall into decay.
語句のかたまりを前から訳していきます。
It is undoubtedly one of the most ancient castles in the kingdom,
それは間違いなく、王国で最も古い城の１つだった
though it was greatly enlarged in later years,
もっとも、それは後年、大きく拡張された
and was kept up until the reign of Elizabeth,
そして、エリザベス女王の時代まで維持された
when it was abandoned as a stronghold,
そして、（エリザベス朝時代には）要塞としては使われなくなっていた
and allowed to fall into decay.
そして、朽ちていくまま放置された
It is undoubtedly one of the most ancient castles in the kingdom, though it was greatly enlarged in later years, and was kept up until the reign of Elizabeth, when it was abandoned as a stronghold, and allowed to fall into decay.
was kept up→ keep up = 持続させる
allowed to fall into decay→（be）allowed to = 〜のままにされる、fall into = 〜の状態になる
それは間違いなく、王国で最も古い城の１つだったが、後年、大きく拡張され、エリザベス女王の時代まで維持された。とはいえ、要塞としては使われなくなり、やがて朽ちていくまま放置された。
It is undoubtedly one of the most ancient castles in the kingdom, though it was greatly enlarged in later years, and was kept up until the reign of Elizabeth, when it was abandoned as a stronghold, and allowed to fall into decay.
when は the reign of Elizabeth（エリザベス女王の時代）を先行詞とする関係副詞です。関係副詞の前にコンマがある非制限用法なので、前から後ろへ訳していきます。
この when は during the reign of Elizabeth という意味です。
動画の後半部分でリスニング用音声が流れます。
