The advanced terrain rules are supposed to be in line with the terrain rules in core rules. These haven't been touched and in are the same as in classic WM.
p. 53
Defended
A stand is defended if it is placed wholly or partially;
behind a low obstacle, such as a hedge or wall; at the
edge of or within a wood; or amongst ruins or
buildings. A stand is also defended if it is positioned
on the crest or upper slope of a hill or rise so that it
occupies higher ground than its attackers.
These are all judged to be situations that confer an
advantage to infantry or to small units of artillery. Such
units can position themselves closely to the lie of the
land, taking advantage of minor folds in the ground,
local vegetation, or barriers such as hedges or ditches.
This advantage is partly due to a unit’s ability to
entrench itself, for individual warriors to find cover, or
for whole units to conceal themselves –for example by
kneeling amongst dense undergrowth. Similarly, it
confers an advantage to a unit that has taken up a
position that is hard or exhausting for an enemy to
approach.
In other words artillery count as defended on the hill if charged or being shot at from lower positions.
