Why are my villagers acting so strangely?

In my survival world, I have a village with a lot of villagers in it. There are two houses, one with 2 floors, and one with 3 floors, but none of the villagers go into the higher floords, just the lower ones. Also, some of the villagers decide to go into the farm, and trample all the crops. A zombie also somehow managed to get on top of the smaller house. I don't know how that happened, but the villagers behaving strangely is my main problem. Is it just that the village itself is too small? Here's a link to a zipped copy of my world: http://www.mediafire.com/file/933lepodb0ea866/Getting%20harder...%20No%20problem%21.zip

Solution 1:

Villagers do not act strangely. They act the way they are programmed to act.
Here is a section of the wiki on villagers about their behavior. It contains a lot of information on how they act.

The villagers generally run to the closest door when night comes. They will usually be out during the day so the closest door is often on the lower floors.

The villagers go into the farm, jump up on the glowstone you are using for light, and then when they jump off, it tramples the farmland. Move the glowstone up a couple of blocks for overhead lighting that they can't jump onto.

The zombies are spawning on the roof tops because you have no lights up there. Add lights.

Here is an image to point out the problems:

I modified the village to reflect these changes: mediafire download of world.
It is named 'Getting harder... No problem! modified.zip' and it is 26.4MB.

Here is a similar image after the changes: