If you choose the prison map and you/your opponent is playing Galactus how many walls do you need to break down in order to move out of the starting area? do you need 2 squares open or 4?

The short answer: Since this creates a special situation that's not covered in any published documents (a map with a starting area that forces a 6x3-square colossal to be placed on squares it couldn't normally occupy), ask your Judges how they will rule.

How I ruled it last week: We had one player using Spectre in Week 10, and one(maybe 2) of his opponents chose the Prison map. As per the placement rules for Spectre, the player had to put 12 of his 18 squares into the starting area, forcing him to 'stand' on the walls there. I ruled that he would not have to destroy any walls to move out due to this unique situation of walls cluttering up the starting area, based on 2 points:

1-Spectre and other 6x3 bigfigs must end movement in legal squares, but it doesn't say anything about how they can start their movement(presumably since the rules assume that they would already be placed in legal squares at the beginning of movement). Once he moves out of that weird prison starting area though, he wouldn't be able to go back in without destroying some terrain.
2-You can start counting movement on any square adjacent to the colossal; and with 6 squares, a full third of his base outside of the starting area walls, he shouldn't have to move 'through' any of those walls. (Truthfully, if we say he must create a space from which he can move out of in the Prison equal to the width of his base{following the General E&C that says colossals require a 'doorway' at least as wide as the base in order to pass through), since he's already wider than the starting area he'd really be forced to destroy ALL those walls before being able to escape... and that makes no sense at all.)

It's unfortunate that we're still dealing with these colossal issues... I mean, there's still errata sitting in the Marvel E&C referencing the old Galactus. I believe WK Game Design is probably clear in what they believe all colossals should be able to do in most situations, but they haven't quite conveyed that over to the rest of us via the rules. In my opinion (very much like the old Galactus errata), colossals should be able to just move right through indoor blocking terrain and walls, leaving rubble in their wake - Galactus is aheckuvalot more likely to pick up a prison and smash it to bits rather than go inside and try to shimmy through the tight spaces.
Personally, I never thought it was a good idea to make colossals usable on forces - such awesome power should really just be relegated to scenarios only, and playing them otherwise brings up all these confusing questions. Oh well, such is war.

Right on. Ok well what is the open squares required for moving them through indoor blocking terraing? Do they need 3 squares open to fit through? On the prison there is 2 square opening on the side, which I don't think they can get through. Is that correct?

I wish I had a prison map to look at right now... but yeah, if it's not a 3-square opening, they won't fit unless a power, etc. allows them to do so (Spectre can Phase, but I think Galactus and Foom would be stuck until they can blast away some terrain).

I gotta say, I'm seriously considering hiding any prison maps at my event to avoid these issues....

Guess I'm bringing my own.... haha

That would be correct. The fastest way a colossal would move on that map is to widen the doorways into the 3 square wide rooms.

Its a weird situation, but the way I see it, that's something in place as a balance so an army of smaller base figs have an easier time moving around.

There was an official ruling on this question. Galactus destroys any walls that he starts on top off.

He must begin a much in the starting zone as possible.

Prison Map + SIF + alternating Barrier = great way to beat Galactus.

if you consider that team a win....

Because playing a colossal is SO much "less cheesy" or "better".

Actually, it wouldn't result in a "win".

If you use the SIF tactic, and it works, and Galactus doesn't kill anyone, but you don't kill Galactus, that would result in a roll-off.

At least you have a 50% chance of winning that match.

I was just considering the barriers as a defensive strategy. From there, you could build a team w/ HSS or similar to slowly wear down the big G.

I have no issues with people using collosal figures. They are legal and if you have one you hardly ever get to use it. I don't even have one and I don't care if people use them. Better than sitting around collecting dust. It is a ton less "cheesy" or whatever than SIF barrier teams.

I think a team with HSS and some support in the prison map will probably be better than just trapping galactus. You could use the other points that were spent on barrier for better figs. But I guess you could just use scarlet witches...