Assuming a 1×1 brick is equivalent to 1 sq foot, and that each brick would then be 1.2 foot tall, I’m guessing ~75k bricks. This assumes ~50k bricks for the walls and ~25k for the rest of the buildings.
The outer curtain, for example, is 300′ to a side, 8′ thick and 20′ tall. That’s ~15k 2×4 bricks. In the real world that would be a bit over 8′ square. It would require 49 48×48 baseplates.
Now that would be a project.
Edit (Monday 2/11): could I really use 2×4 bricks? Each of those would represent a near impossibly heavy 2′ wide, by 4′ long, by 1.2′ tall rock. 9.6 cubic feet or about .27 of a cubic meter. If a cubic meter of rock weighs on the order of 2500kg, each of those innocuous looking 2×4 bricks would be the equivalent of about 1,500 pounds (more or less). Even a 1×3 brick would likely weigh about 500 pounds, so figure a 4-6 man crew working it into place. Now, admittedly, when one is planning on recreating a fictional castle using 21st century toy plastic blocks, unassailable realism does not need to be our first concern. Still. Something to think about.