Are monsters destroying my walls?

Solution 1:

As mentioned in the comments, it sounds like an Enderman has come to visit. These creatures spawn in darkness and (amongst other things) will pick up certain blocks and move them around, as described on the Minecraft Wiki:

Endermen have a unique ability to pick up certain block types and move them around, which they generally spend their time doing while neutral. They can also step up one full block without having to jump.

In terms of defending your home from Endermen, the answers to this question have some tips you may want to try. Alternatively, you can try building your home (and surrounding wall) out of materials that an Enderman can't move, such as stone or brick (the Wiki article has a full list of blocks that they can move.)

Solution 2:

A little addition to what was already said: Another possibility is to cover the wall in water. Endermen will be hurt and flee if they touch it, therefore unable to touch your wall.

Solution 3:

The Enderman is a semi-passive mob that has a somewhat annoying ability. They will actually pick up and re-place blocks (generally just dirt blocks) as they see fit.