You'd think you'd unlock the doors, but when zombies are around, you're more likely to want to add more on. This goes extra-true if there's an Abominalpha running around. Both of these things are the case over in the Zombicide: Black Plague Kickstarter campaign. Continuing to surge right along, they've unlocked two new stretch goals (Thundergut and Abominalpha) and have added on new 3D doors as an optional buy.
In a game like Zombicide, you've got a flat board with flat terrain on it, but lots of wonderful figures in the form of Survivors and Zombies. Now, personally, that's how I like playing games. I like a good mini, but I hate when terrain gets in the way of where I should be able to put them. But a lot of people feel they miss out a bit with all those flat pieces around. The 3D doors set alleviates some of that, by giving you a better representation of the doorways in the game. You can even clip little color-code markers at the bottom so you know what color door is what.
If you've not joined in yet, you still can for the next 10 days.