its not about the depth, there no needs for depth in an essay....i quote my teacher here...........'there must be a clear line of argument'.
as for revising for it, i would suggest not only just know the facts, but look at the process of change and how the event came about, the long-term and the short-term factors.......look at the consequence of change...........look at the continuity of political parties/party politics....etc. hope this helpful in some way..
from my experience, i knew all my facts, what i messed up was my essay, i didn’t give both sides of the argument, which i realised after the time was up!