Well, hello, this is my first article. This engine is not perfect but I just discovered how to make it, and it is pretty good. Maybe some of you already know it but anyway here it is:

The lemmings must have platform movement, with no speed, acceleration nor deceleration. The floor and walls are obstacle backgrounds.
The events are:

1. Lemming collides with the background: Stop
2. Lemming is stopped
+ Lemming's internal flag 0 is on: Set Xpos of Lemming to Xpos of Lemming + 1, Set direction of Lemming to ->
3. Lemming is stopped
+ Lemming's internal flag 0 is off: Set Xpos of Lemming to Xpos of Lemming - 1, Set direction of Lemming to <-
4. Lemming is over an obstacle background
+ Lemming is stopped: Toggle internal flag 0

And that's it! It's pretty simple, since they can't go up slopes and stuff, but works well, and if you're going to make a game with it, you can improve it as you go, like adding falling animations, the thing from where they come out, etc.