Recently I encountered two occasions of seasoned players treating ruins incorrectly. Mostly I chalk it up to erroneously remembering back to 4th edition, but the ruins rules do admittedly require a lot of good sportsmanship and reasonableness. They even contradict themselves in places, for example noting that measurements should be done from base to base although every example and rule in the section measures from anywhere in the model (coherency) or in the abstract along the 2D plane (templates). In any case, it’s worth recapping a couple points about ruins:

• Players should agree beforehand whether the ground floor of the ruin is area terrain or not, particularly if it doesn’t have a base (the rulebook defaults to no in this case). Personally I think it makes the most sense to treat the ruin’s entire base as difficult and area terrain if it has one, but I often encounter players who only wish to treat the area inside the virtual perimeter of the ruin walls themselves as such.
• One of the biggest points of contention or incorrect assumptions I frequently see is whether or not ruin walls are impassable or simply difficult. The rulebook defaults to the latter, but many players and groups default to models not being able to pass through them. This is an important point that should be clarified at the start of any game, as it can dramatically change movement around the board.
• Unless players agree otherwise, when moving in a ruin, you don’t move models the straight line 3D distance to where you want them to go. Instead they move in a sort of z-axis Manhattan distance, over the horizontal component, and then up the vertical distance. This is made very clear on page 83 in the big book. So, for example, to assault a model over two inches looking down on the table top and up a level (three inches), a model would have to make a difficult terrain roll of 5 or better. Levels should be assumed to be 3″ tall unless players agree otherwise.
• Models can only go onto a level in a ruin if they can physically fit there, and even then some models such as beasts and bikes cannot go up levels. However, models can assault models even if they cannot physically be placed in base to base, provided they could otherwise make the distance. For example, if it has the distance to move up the level but does not fit, a model could be put directly under a model on the next level down and be assumed to be in base to base.
• Barrage weapons always hit the highest level under the marker’s center hole after scatter. When firing template and non-barrage blast weapons you must declare a level that they’re targeting.In all cases, only models on that level may be hit by the weapon, though casualties may be removed from anywhere in the unit as usual. With template weapons, you generally may pick the same level or the one above or below the firing model, however, skimmer, jump infantry, and jetbikes may target any level. Note that the rulebook shows a template being measured purely in the 2D plane above the model, indicating that it does not need to lose those precious inches going up or down a level.
• All Independent Characters and Monstrous Creatures have Move through Cover, so they roll 3D6 when moving through ruins on their own (or with units that also have Move through Cover).

Again, ruins require a fair bit of sportsmanship to deal with well but it helps for everyone to know the current rules to begin with, and to talk for a moment at the start of each game and make common assumptions explicit. Cover values, ruin bases, and moving through walls are the three things I ask about first before doing anything else in each match.