One way to hold this word is to keep the small word fort inside it, together with the long ending that means the making of. So the picture is an action: something already standing is given more, so that it can take more. Give a town walls, towers and a ditch, and it can hold out. Give flour iron, or milk vitamin D, and it can feed people better. Give a river bank more earth, and it can meet the rain. The word then does one more thing, which is common in English: it slides from the work to what the work leaves behind, so the walls and towers themselves are called fortifications.
From the word’s history
Wiktionary traces fortification through Middle French and Late Latin fortificatio to Latin fortis, strong, and analyses the modern word as fort plus -ification. That root of strength is what both uses in this entry come back to.
Wiktionary 把 fortification 經中古法文、後期拉丁文 fortificatio 追到拉丁文 fortis（強），並把現代這個字拆成 fort 加 -ification。這個「強」的字根，正是詞條裡兩個意思共同回到的地方。
packet evidence: Wiktionary etymology_text for fortification (source_ref kaikki:3f6f69a190036ae4:642712416:24008)
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