SUV0508
I would like to quote Aristotle's SC Grail:
"Since Like is a preposition and As is a conjunction, use Like only to compare nouns and As for all other".
Although we are comparing the Acropolis to a fortress, we should have used Like. But why does this rule doesn't apply here?
[Yes I know putting Like in place of As sounds a bit odd]
What's going on is that the sentence does not compare the Acropolis with a fortress. Let's take a look at the correct version.
The Parthenon was a church from 1204 until 1456, when Athens was taken by General Mohammed the Conqueror, the Turkish sultan, who established a mosque in the building and used the Acropolis as a fortress.Notice that "as" is used in a "used x as y" construction, to convey that General Mohammed "used the Acropolis as a fortress."
A "used x as y" construction does not express a comparison. Rather, it communicates what something was used as. To communicate that one thing served as or was used as another, we use "as" rather than "like."
Here's another example:
As superintendent of the school system, John Andrews achieved many laudable goals.
In that sentence, "as" is used for communicating that John Andrews served as superintendent.