How can I control where enemies place troops?

As jco mentioned, too much space between your structures (including walls) will cause a gap. If you move any structure (doesn't have to be in edit mode), you will see a white light outlining your base. Troops cannot spawn in this area.

I moved a structure to see the property line and noticed where a gap was:

After filling the hole (normally I have my archer tower there, but this just for demonstration):

If the gap is 2 squares or greater, troops will be able to spawn here.

Note: Traps do NOT fill those gaps.

Too much space between your structures (more than 2 squares) will cause a gap which can allow enemy troops to be placed inside. Try moving structures closer together or fill any gaps. I believe edit mode will show you these gaps.

Enemy troops cannot spawn within 1 space of your buildings except for traps (this includes hidden tesla.) Your walls have a gap on the middle of them 1 space from your walls. To fix this simply re-arrange your setup to include no gaps.