I have a throne in my adventure map, and there will be the king zombie on it. But it doesn't look good if the king zombie is only on the left block, so how do I summon the zombie in between the two blocks on the throne?
/summon command automatically adds 0.5 to the spawn location, so that they spawn on only one block. You can, however, override this by adding .0 to the X or Z coordinate.
Example of what I mean:
/summon Zombie X.0 Y Z /summon Zombie X Y Z.0
Replace X, Y and Z with your own coordinates. I'm not sure what your coordinates are, so try them both.