Monsters
The wizard mentioned an infestation of vile goblins, so let's add them in.
I'm using this goblin, but use whatever you like...
Drag an instance of the goblin to a suitable location, resize it, then set an appropriate animation (...I'm using animation 1, Run).
I renamed the object to GoblinRec - all the Goblin instances from this object will have the following Rectangular Movement pattern.
Monster Behavior
For this game, we'll only have a very simple monster behavior. The monster will move in a perpertual loop, and won't attempt to chase the player.
Rectangular Movement Behavior
Double-click on the monster, and go to the Behaviors tab.
Click Add a New Behavior.
The one we want is already installed by default, so we just need to search for Rectangular Movement.
When you find it, click and add it.
You need to choose the height and width bounding the rectangular movement, as well as the acceleration and speed. Initial position and whether it is clockwise or not will also effect the movement.
Play around with it until you find a suitable set of values.
Movement Events
You can use events to flip the character when they move in the opposite direction, like so:
Collisions
For now, we can just make collisions with the monsters restart the level:
Later on we will be creating a Game Over scene we could use instead.
We could also add Health to our player, and coliisions may affect that instead.
Moving In a Single Line
You can use the same Behavior to create a 1-dimension movement pattern for monsters or obstacles, by either Zero-ing the horizontal or vertical speeds.
Instance-Specific Control
You may wish to use the same pattern, but with different properties. You can either create complex instance-variable control (set and use instance variables in the movement - e.g. set Behavior X Distance = Goblin.instance.Variable(XDist))
Or, you can create many different Objects with varying Behavior properties - GoblinRec2, etc...
You can even use random number generators to create various unexpected patterns, or non-repeating behaviors.
For more Monster control, please look for the Monsters Revisited page.