They showed me that the wild trees I had seen coming up the river were left there merely as a protection to the garden, and I saw that the stockades which surrounded the island were so placed as to prevent any persons landing, and might serve also as an outer fortification to the castle. (In the Wilds of Florida by W.H.G. Kingston） 注：castle = 大邸宅
動画の後半部分でリスニング用音声が流れます。
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。
They showed me／（接続詞の前）that the wild trees I had seen coming up the river／（長い主部の後）were left there merely as a protection to the garden,／（接続詞の前）and I saw／（接続詞の前）that the stockades which surrounded the island／（長い主部の後）were so placed as to prevent any persons landing,／（接続詞の前）and might serve also as an outer fortification to the castle.
語句のかたまりを前から訳していきます。
They showed me
彼らは私に教えた
that the wild trees I had seen coming up the river
私が川を上りながら見た野生の木々は
were left there merely as a protection to the garden,
ただ庭を守るためにそこに残されている
and I saw
そして私はわかった
that the stockades which surrounded the island
島を囲んでいる柵は
were so placed as to prevent any persons landing,
誰も上陸できないように配置されている
and might serve also as an outer fortification to the castle.
そして、大邸宅の外側の防御策としても役立つかもしれない
They showed me that the wild trees I had seen coming up the river were left there merely as a protection to the garden, and I saw that the stockades which surrounded the island were so placed as to prevent any persons landing, and might serve also as an outer fortification to the castle.
I had seen coming up the river→ 動詞のing形が「〜しながら」という意味になることがあります（コンマのない分詞構文）。coming up the river：川を上りながら
as a protection to the garden→ as = 〜として。「庭の防御として」→「庭を守るために」。文末の as an outer fortification to the castle の asも同じ意味です。
私が川を上りながら見た野生の木々は、ただ庭を守るためにそこに残されているのだと、彼らは私に教えた。そして、島を囲んでいる柵は、誰も上陸できないように配置されており、大邸宅の外側の防御策としても役立つかもしれないということがわかった。
今回の英文は前半と後半で同じ構造が使われています。
They showed me that the wild trees I had seen coming up the river were left there merely as a protection to the garden, and I saw that the stockades which surrounded the island were so placed as to prevent any persons landing, and might serve also as an outer fortification to the castle.
前：They showed me that〜→ 彼らは私に〜を教えた
後：I saw that〜→ 私は〜がわかった
前：that the wild trees I had seen coming up the river→ the wild trees（I had seen coming up the river） カッコ内の語句が後ろから先行詞を修飾。trees の後ろの関係代名詞が省略されています。
後：that the stockades which surrounded the island→ the stockades （which surrounded the island） カッコ内の語句が後ろから先行詞を修飾。
★ might serve also as an outer fortification to the castle の主語は the stockades です。
