Think of a thick wall of packed earth and tree trunks, built low and heavy, with the danger on one side of it and you on the other. It is not a tall, thin wall for looking over — it is made to take a blow and stay where it is. Everything this word does comes back to that: something solid standing between you and what is coming.
From the word’s history
The packet's Wiktionary extract traces bulwark to Middle Dutch bolwerk, which it analyses as bole (tree trunk) + work, and notes it is a doublet of boulevard. So the wall of tree trunks is the literal starting picture, and boulevard is the same word borrowed through French for the road laid where such walls once stood.
來源的 Wiktionary 抄錄把 bulwark 追到中古荷蘭語 bolwerk，拆成 bole（樹幹）＋work（工事），並註明它跟 boulevard（大道）同源。所以「用樹幹堆起來的防禦牆」是字面的起點；boulevard 則是同一個字經法文借過來，指舊城牆拆掉後改成的那條路。
Wiktionary extract supplied in packet (kaikki:3f6f69a190036ae4:507748409): 'From Middle English bulwerk, from Middle Dutch bolwerk … equivalent to bole (tree trunk) + work … Doublet of boulevard.'
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