What finer antagonist than the humble Kobold? Select Blueprints > Creatures > Humanoids > Kobold. Now, wherever you click, you’ll place one of the horrible creatures. Place a group of four at the opposite end of your valley to the player start position. To move them once placed, select the Select Objects option from the bars at the top. To arrange them more attractively, you can rotate anything you’ve placed with Shift + Right Mouse.