光緒21年（1895）10月15日，日軍「吉野」等艦進攻打狗港，這次戰役中，旗後砲台門額前二字被日艦擊毀，只剩「天南」二字。本文經比對墨跡，發現旗後砲台門額的題額者並不是沈葆楨；從文獻資料比對，推論丁日昌與岑毓英也不是題額者。題額者不外乎淮軍官員或外聘文人，由於缺乏史料佐證，已經難以考據。日治時期迄今，旗後砲台門額題字有：「威震天南」、「威振天南」、「威鎮天南」、「砥柱天南」等四種說法。「威震天南」說法有誤，因其引用時將「威振」誤植為「威震」。「威鎮天南」與「砥柱天南」之說，並沒有交待門額題字的來源或依據，真實性存疑。「威振天南」之說，源自日治時期吉田寫真館發行的明信片，以及陳錫如的〈旗山記〉，這二個文本都沒有交待門額題字的出處，真實性有待查考。因尚未找到明確證據，旗後砲台門額題字為何？仍是未解謎團。

Imperial Japanese Navy attacked Ta-kou Port on 15th October 1985. In this battle, first two Chinese characters on the inscribed board of Chi-hou Fort were smashed by naval vessels of Japan, only last two Chinese characters - ＂Tien-Nan＂ (天南) left on the inscribed board. It is clear that those characters on the inscribed board of Chi-hou Fort were not writed by Shen, Pao-chen (沈葆楨) after calligraphy examination. Ting, Jih-chang (丁日昌) and Tsen, Yu-ying (岑毓英) did not write those characters either, which was inferred from textual analysis. Those characters on the inscribed board might be writed by the army's officers or engaged scholar of Ching Dynasty. It is hard to confirm due to lack of evidence. There are 4 ways of saying regarding Chinese characters on the inscribed board of Chi-hou Fort up to date, that are ＂Wei (威) -Chen (振) -Tien (天) -Nan (南)＂, ＂Wei-Chen (震) -Tien-Nan＂, ＂Wei-Chen (鎮) -Tien-Nan＂ and ＂Ti (砥) -Chu (柱) -Tien-Nan＂. The saying of ＂Wei-Chen (震) -Tien-Nan＂ is wrong due to incorrect quoted from literature. ＂Wei-Chen (鎮) -Tien-Nan＂ and ＂Ti (砥) -Chu (柱) -Tien-Nan＂ are disbelief because of no reference source. The saying of ＂Wei (威) -Chen (振) -Tien (天) -Nan (南)＂ referred from a postcard of Yoshida photo studio (吉田寫真館), as well as a article of Chen, Hsi-ju (陳錫如) entitled ＂A travel to Chi-shan＂ (旗山記). Neither postcard nor article mention explicit information sources, the authenticity of this saying still need to prove. Therefore, throse characters on the Chi-hou Fort’s inscribed board is still a mystery as a result of no hard evidence has been found to confirm this saying.