When I resize the window (to 640 x 480, exactly two times the size of the game) it kind of blurs the pixels, which totally ruins the retroness. Any way around this?
Edit: Oh and yes, sampling is set to point in case you were wondering.
To keep point filtering, instead of stretching the window with the window object, you will have to zoom each of your layers 200% for x and y.
But there's a problem... you also need to make your layout bigger around the edges to compensate for this. For 320x200 scaled up, you will need an extra 160 pixels at the top and bottom of your layouts, and an extra 100 pixels on either side, to make a buffer of sorts. Otherwise those areas won't be visible when you zoom in. But this will respect point filtering.
Of course, you could avoid the pixel boundary problem by setting your layout to "Unbounded scrolling," but the character will always be in the center of the screen, and you have to add extra decorative graphics around the edge of your layout anyway because the game will scroll outside the boundaries.
You might consider just making your sprites and tiles all scaled 2x before importing them into construct and use an unscaled resolution of 640x480. Sure, it will appear to have sub-pixel movement to the trained eye (because every pixel is actually made of four pixels) but you still get that chunky retro look without the headache of doing any of the above. This is how I'm doing my game.
It's really up to you though. No way is perfect, so you have to decide which of the evils are lesser enough to still work with. I think Rich or someone mentioned something about point filtering when stretching the display recently but I can't remember, I just woke up and my head is all fuzzy.
Hope this helps.