Simple, is it legal to move through windows in the same way as moving over obstacles smaller than your sillhouete?
There might be an official answer here, but as a TO: always discuss terrain rules with your opponent before a game.
Terrain rules are poorly defined/enforced. If you don't specify anything, a window is just a hope in the terrain and you'll have to be able to fit your silhouette through it standing in order to get through. Personally I prefer to treat them as narrow access points which ends your movement directly on the other side.
Local communities also often have custom terrain rules that best fit the terrain they play with locally.
@ijw Any recommendations? Also, in the wiki there is no clear explanation how passing through a narrow gate is actually done...
Each groupe I play with have different way to handle it. I used to consider you can move though just like you can vault over obstacles.
As a TO, I use to write the special rules applying to each table. For example, you cannot pass or shoot through window glasses (they're armored ones). No shoot / engage AROs. But you can pass through opened windows with halved move (as if you were prone).
That's because there are no special circumstances associated with it other than that it's physically impossible to stop halfway through. You'll find more precise wording under destroyed scenery (basically you just place the model on the other side in base contact with the narrow gate, and remembering the basic movement rules also gives you that you have to pay for this movement)
This is the RAW. As with other terrain rules, you can decide with your opponent beforehand to do whatever.
One of the good (RAW) options is to use the structure rules and designate Windows as... Windows with Narrow Access. (Related rules: http://infinitythewiki.com/en/Scenery_Item_Profile_Chart ) This makes them a Narrow Access. Which means that S2 or smaller troopers can fit through them. If I'm doing this I further make it clear that they're at the height they are on the terrain (this means that, usually, S0 can't fit through them). I find that this turns a lot of 'single door' buildings into less of a cul de sac, but still limits the movement of many troops.
It's worth noting with this that doorways typically have doors and not specifying windows open/closed can mean weirdness where a window is always going to be faster and mmore convenient to get through. For this reason I personally prefer windows to act like holes blown open in walls by missile launchers (i.e. kind of like heavyweight); movement ends on other side
I usually run this on tables without doors. And yeah, classing them as "Difficult Terrain (Undefined)" isn't a bad option either. It puts them about on parity with Doors for getting through and rewards Terrain Skills. Scenery rules you very much need to play on a table by table basis.
Technically it can be faster to enter/exit a building through the window, but it tends to make you unpopular with the owner and you'll rarely get invited over for tea again.
For other things that make you unpopular with the owner, see also: * Starting gunfights * Using bulk explosives * Insisting on soy milk in your tea
Some people are genuinely lactose intolerant so I'd never begrudge them wanting to avoid a bathroom emergency due to milk in tea. They will have to put up with me calling it "Soy juice" though, as by definition it is not milk.
Fun thing about windows: window glass is actually made out of several large and very sharp throwing daggers, all thrown in your general direction at roughly the same speed as that which you moved through said window. Also, window glass is kind of hard and shatters only when inconvenient.