Hi, again, Magdalena, this time in English. For start, this is not ancient Greek. I'd call it παλαιά καθαρεύουσα, but in no way ancient. It might sound uneeded but I can't see why a professor would not help you with tough, unknown words.... I am only surprised, that's all.
So, I gave it a go. In case you don't know, the txt is a small treasure, first published as a book in the late 19th century. It is a historical text that refers to facts that took place during the years that Greece belonged to the Ottoman Empire.
(This is) About (regards) Mehmet Sultan. When Mehmet Sultan came to power, ruling both the West and the East made a harsh decision against Constantinople and the Christians living there. Although, seemingly he showed great love to its (Constaninople's) citizens, it was a cunning one and bore malice towards all. At a place over the lighthouse (after a long search, I believe the text is talking about the lighthouse in Bosphorus) known as (the) 'murderer' he built a fortress (<em>although the Greek word is "κάστρον", it is referring to the fortress known as "Rumelihisarı" in turkish or "Rumelian Castle and Roumeli Hissar Castle" in English. For more, pls check this link: http://en.wikipedia.org/wiki/Rumelihisarı. It'll help you understand a lot about the next. Also, in my opinion, there is a slight chronological inconcistency, having to do with when the fortess was built. Historically, we know that Mehmed built it before conquering Constantinople</em>.) with strongly founded towers, the so-called 'new castle'. During its construction, the citizens were providing (the builders) with food and stones because they were afraid of him fighting against them.
Let me know if this helped you and whether you have more questions regarding the translation or meanings of the text. I'll do my best to give a hand once more.
Λήδα