[PC Windows]
Level select:

Use the following trick to unlock all the levels in the campaign mode. Note: This procedure involves editing a game file; create a backup copy of the file before proceeding. Use a text editor to edit the "*.usr" file in the folder with the profile and save game directories. The *.usr file contains information about the missions and other game options. You will find some lines like this:

*VARIANT 1 0
*VARIANT 2 0
*VARIANT 3 0

The first number is the difficulty of the stage, while the second one describes its
completion. To unlock all the levels, edit the lines to resemble:

*VARIANT 1 1
*VARIANT 2 1
*VARIANT 3 1

[PC Windows]
Invincible Troops:

This works on all but single man troops. When the troop is almost out of the village, highlight it with the highlight window, and divide the troop of press S. One half of the troop will be normal, and take damage, but the other half will be invincible.

Note: This works even when the troop is demoted.