A pretty common mistake, especially amongst players that have been around for multiple editions of the game, is to forget some of the basic movement and placement quirks in 40k. This is compounded when you have to read multiple rules in conjunction with one another. Let's look at movement within a building, for example.
The rules are very clear in the rulebook regarding how a model may move in relation to other models:
"A model may not move into or through the space occupied by another model (which is represented by its base or hull) or through the gap between friendly models that is smaller than its own base (or hull) size. A model cannot move so that it touches an enemy model during the Movement and Shooting phases - this is only possible in an assault during the Assault phase. To keep this distinction clear, a model may not move within 1" of an enemy model unless assaulting." (Page 12, BGB)
Unfortunately, sometimes things can be confusing when you through a ruin into the mix. Part of that problem stems from the fact that you can kind of fudge the distances between models for the purposes of unit coherency and the assault phase.
"In the course of movement, it is possible that several models from the same unit may end up spread across two or more levels of a ruin. When this happens, the models in the unit maintain unit coherency as long as any part of the body of a model on a lower level is within 2" of the base of a model that is higher up." (Page 82, BGB)
"In some cases the ruin might genuinely be unstable or uneven, or the space could be very limited on a particular level, making it impossible to move assaulting models into base-to-base contact with the unit they wish to assault. When this happens, it is perfectly acceptable to place models as close to their foe as is safely possible, including the level below or above, providing that you place the assaulting models as close as possible to their opponents and you make very clear to your opponent which of your models are in base-to-base contact with his models. We find that directly below or above works well, representing them charging up or down a flight of stairs." (Page 85, BGB)
The problem that people sometimes forget is that there are only two instances in the rulebook that allow you to "imagine" that a model is able to cover distance that he, in fact, cannot physically cover. Because of these two exceptions, it is easy to assume that you can walk between floors despite the fact that there is an enemy unit taking up all the floor space above you. Remember the cardinal rule on movement in ruins:
"Only certain troops are capable of clambering to the upper levels of ruins. Accordingly, only infantry, jump infantry, jetbikes, monstrous creatures and walkers may move on the upper levels of a ruin - and only if the model can physically be placed there." (Page 83, BGB, emphasis supplied)
If moving on or in between floors would take you within 1" of an enemy model, you just can't move there, plain and simple. And while this may be hard for some folks to swallow, it certainly isn't without precedent. Look at the image below.
If this is taking place out on solid ground, outside of a building, the enemy can certainly prevent you from getting somewhere by locking elbows and physically taking up all the space. Area denial is a pretty basic tactic! If you can do that on open ground, doesn't it make even more sense that an enemy unit would be able to do it within the confines of a building where there is even less space to maneuver? If Mr. Blue wants to get from point A to point B, and the Red Unit is in the way, he is either going to have to jump/fly over him, or assault through him. In a building, the rules conveniently allow you to do just that when there is no actual space on the floor to get models in base-to-base contact. It's almost as if they planned it that way...
Another quirk that the ruins rules offer us is in use of template and blast weapons. Just remember that there are three rules regarding templates depending on what kind of template you're using:
(1) Template Weapons: Basically, I mean flamers. Templates/flamers can only affect models on the same level, and they can only be fired on the same level as the firer, or on the level above, or on the level below.
(2) Blast Templates: Doesn't matter if you mean the big one or the little one. When you fire a blast weapon, declare which floor you're shooting at, and then roll for scatter as normal. The trick here is that the blast weapon can only affect models on the floor you've declared. If it scatters out of the ruin or is not on any model occupying that level (even if models above or below are covered), then it's a miss.
(3) Barrage Weapons: Note this only applies to weapons that have the "barrage" ability in their description -- some weapons, even large ordinance weapons, are just blast weapons unless it actually says "barrage" in their description. Barrage weapons are considered to be "lobbed" high into the air and then dropped on their target. Accordingly, once you've rolled for scatter, look at the hole on the template. The template is always considered to strike the highest level that is under the hole (even if it is split over two different levels), and only models under the template and actually on that highest level are hit. You don't strike everything at that level and down -- these are highly specialized munitions... not water balloons.
And that's about all I've got to say on the matter at this point. I'm sure I'll come up with other clarifications to post at a later time (because I'm always managing to screw something up in a game), but I think that's plenty for now.
