You'll get straight vertical lines if you keep the camera level. Tilt it down and the lines will start to converge. That's normal. If you had a camera with tilts and swings you could change the angle of the camera back and lens board or raise or lower the lens while keeping the lens board parallel with the vertical to achieve a look up or look down that maintains vertical lines. There is no way to do this easily in After Effects.
Your best bet is to parent the camera to a camera controller null placed at the same position as the camera and then move the null without changing the X angle. You could also just use a single node camera and maintain a level camera. This will allow you to move the camera up or down in the scene while keeping the lines vertical. You cannot, however achieve the same effect that you would if you had a camera with tilts and swings.
Here's a sample comp with a simple rig.
Thanks Rick - that actually helped...
I know what you mean with the tilting, but i kint of wanted a "top down view" to be able t see the rooftops. In your example file only the "houses" in the front show their tops.
So here is my workaround:
I doubled the composition height, so I could move all the houses in the bottom.
Put this composition in another with the actual movie size - et voila I got my look and i can still manage tilts on th Y angle