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.
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)
Two branches
把東西弄強的過程
這是字面的一層：fort（強、堡壘）加上表示過程的尾巴，講的就是「加強」這件事本身。
noun · C1
The act or process of making something stronger: building walls and other defences around a place so that it can be defended, or adding something to a product, such as vitamins to food, so that it does more good.
強化；加固（過程）
加固之後留下的東西
同一個字從「工程」移到「工程的成果」：牆、塔、壕溝就站在那裡，看得到也走得上去。
noun · C1
Walls, towers, banks of earth, ditches and other strong structures built round a place to protect it. In this meaning the word is used in the plural.
防禦工事（城牆、塔樓等）