Considering ravell's advice I may be on the right way (I estimated something around 1800 buildings inside the city walls).

@Moe
Unfortunately there's not much to read about Merridia until now. If you're interested still: Merridia or, partly this thread: Weltgeschehen - Nordreiche (yes, it's german).
19 inhabitants/acre are too few. I'll have a try at a hopefully more exact calculation using your numbers:
The space used by buildings (without castle, temple and harbour) is 110,000px at a scale of 1px = 1m (-> the original picture, I let Photoshop count them), so 110,000m˛ or 27.2 acre. Right? I'm really no genius when it comes to mathematics...
So there would be 5440 inhabitants (assuming 200/acre)... I'm confused :/

Both calculations hint at an incorrect scale, am I right? Using an scale of 1px = 2m would double the number of inhabitants, I suppose?
Sorry for my dumbness concerning numbers

edit: Thanks for the link, Moe, seems useful & I'll download it!