When targeting a zone border, one of the two zones adjacent to the border must be within range of the spell, and the border itself needs to be in Line of Sight. So, it doesn't matter that you have Line of Sight to the zone containing the Steep Hill, what matters is if you have Line of Sight to the zone border. Line of Sight and Range are checked separately. Hagen is completely right on this.
So, Steep Hill blocks Line of Sight if it passes through two "sides" of the zone, meaning the question is whether or not it passes through a second "side" of the zone before reaching the Zone Border. Now, the "side" of a zone isn't a game term, so we just have to trust common English a bit with this, but I think it's fairly clear that the "side" of the zone is synonymous with Zone Border. They are in actuality the same thing.
This would mean that when you trace Line of Sight, you are getting to and touching the second side of the zone, BUT you are not passing through it. So you would be able to target any of the zone borders adjacent to the Steep Hill. Which are, as iNano and exid pointed out, vulnerable to range 2 attacks in the example setup, like those from Akiro's Hammer, because again Range and Line of Sight are checked separately.
Edit: Just to remind people of the rules, for those saying that two of the borders would be visible but the far one wouldn't be, Line of Sight is traced from the center of the zone to the center of the zone border. So all three questioned borders are identical in the presented problem, with Line of Sight having to first pass through one border and then touch the middle of the border in question. Whatever answer applies to one, applies to all three. You can't skew the origin or the destination to try and circumvent obstacles.
Apologies for raising this thread from the dead, but this situation came up in a game tonight, and we fell to arguing.
The result we came up with opposes Zuberi's reasoning here.
The problem is that the rules for LoS as written have not taken into account any possibility that something other than a wall can block LoS:
"Some walls block LoS. You cannot cast a spell or make a ranged attack through a wall that blocks LoS. . . Draw an imaginary straight line from the centre of the attacker's or caster's zone to the centre of the target's zone. LoS is blocked if the line crosses through a zone border that has a wall on it with the blocks LoS trait. . . When drawing LoS to a wall, draw it to the centre of the zone border."
That's what we've got to go on.
The Steep Hill is a different beast. It sits within the zone, not on a zone border the way a wall does. It says:
"LoS that passes through two sides of this zone is blocked."
The language on the Steep Hill card is loose and imprecise. However, it is clear that because Steep Hill sits within the zone and not on its border it does
something different to a wall and the rules as written on LoS (which talk strictly about conjurations on zone borders) cannot completely apply.
Because "LoS that passes through two sides of this zone is blocked" the Steep Hill needs to be envisaged as a raised platform with straight up and down sides. Since LoS goes from centre to centre there isn't any virtual 'gentle slope' you can peek over to look into a diagonally adjacent zone.
Simply put, if you are standing in a zone adjacent to a Steep Hill you do not have LoS to any zone border other than the one directly in front of you,
because there is a large mass of earth sitting in the middle of the zone.
It has been argued that walls, sitting on a zone border, do not "cross" a border, but clearly the rules on range show that walls belong to, and affect, both bordering zones and thus casting a wall must necessitate "passing" the zone border in some way. (You might also say that since you can't have two walls back-to-back against each on either side of a zone border a wall must have a foot in both camps, as it were.)
If you think of it like that, then you need to cross two zone borders to cast a wall along the far or side zone borders of an adjacent zone (even if only by a little bit).
I am usually all in favour of taking rules-as-written, but in this case I do not think the rule has been written. LoS as it refers to seeing through walls is certainly there and is very clear, but there is nothing other than a fairly vague one line for a LoS blocker that sits within a zone. Rules-as-intended need to have the casting vote.
tl/dr: You can't build a wall on the other side of a Steep Hill, because you can't see the zone border as there is a Steep Hill in the way.