Rookie or not, when it comes to the Stronghold series, there are tough fights ahead and many times one single unit can make a huge difference.
For this reason, trainers such as this one provide a helping hand by allowing you to heal a selected unit.
To do so, all you need is to run the trainer, launch the game and while you are in the Main menu section, press the F1 hey to activate the trainer. After that, selecting a single unit and pressing the Numpad 7 key will heal or debuff it.
NOTE: This trainer is for the version of the game.