WARNING: This post is only for those who enjoy the intellectual stimulation associated with being a rules-lawyer!
Two issues:
1) Checking my analysis of the RAW to ensure that large monsters cannot expand into obstacles.
2) I can't find any rule that actually says monster figures must be be placed in an empty space.
EXPANDING INTO OBSTACLES
I have done a ton of searching on this, and while I see people assert that large monsters cannot expand into obstacle spaces, I want to make sure the rules actually say this.
To be clear, I don't think you can do this, but I want to make sure the rules actually say this.
I think the applicable rules are the following:
1) "Obstacle spaces are indicated by a red line surrounding them. Obstacles are considered to be blocked spaces, they block movement and line of sight." (CRRG p. 38)
2) "Figures cannot move into or through spaces containing figures or
obstacles." (Core Rules, p. 8)
3) "A large figure expanding into a space is not considered to be entering or moving into that space." (CRRG p. 21, 25)
4) "A figure may not end its movement on a blocked space." (CRRG p. 25)
The core of my question is, "What does "end its movement on" mean? To figure this out, we ask "When does movement end" and "what does 'on' mean in this context?"
I believe "on" means "if any part of the base of a large monster occupies that space."
That leaves us with asking "OK, so when does movement end?"
Regarding when movement ends, the CRRG says,
"When ending or interrupting movement, the active player places the figure so that one of the spaces that its base occupies includes the space where the figure ended its movement (“expanding” it)."
This is the wishy-washy part. It doesn't explicitly say whether expansion happens before or after movement end. It just says "when ending movement, expand the monster." Given that it says you expand a moster "when" ending movement. I suggest that there is a "ending movement" phase that goes something like this:
1) Declare that your are not moving any more or that you are interrupting movement.
2) If moving a large monster, expand it.
3) Movement end.
I think this completes the rules needed to forbid expansion into obstacles, but I want to make sure I wasn't missing a simpler way to get there like some rule that says "large monsters can't expand into obstacles." :-)
PLACING MONSTERS ON OBSTACLE SPACES
Now for the real head-scratcher: Can anyone find a rule that says you can't place (using the formal game rules definition of "place") monsters (even small ones) on obstacle spaces?
The closest I can find is an implication that you can't:
"Monsters are placed on the map during setup, as reinforcement
or by other game effects according to following rules:
––Individual monsters or monster groups have to be placed in such a way that the rules for its placement in the quest description are fulfilled to the greatest possible extent.
–– If monsters cannot be placed as indicated because there are
not enough empty spaces available, they are placed in the
closest empty spaces instead."
While the highlighted language implies that you can only place monsters in empty spaces, it does not explicitly state that.
It isn't forbidden via movement rules, either:
"If a figure is being placed from off the map, it is not entering a space. This includes heroes being revived or standing up, reinforcing monsters, summoning familiars, etc." (CRRG p. 25)
I find it surprising that nowhere in the rules does it say "When placing figures, they must be placed in empty spaces."
Am I missing that rule or do we just go with the implication included in the above-cited rule?
Again, this is just an exercise in the problems with writing clear rules that cover everything. Pay me no mind if you don't enjoy such exercises!