You can define a path - say, with small invisible sprites for each point. Use MoveTo behavior to move from one point to another. Set angle to angle(startX, startY, endX, endY)+90 when the enemy is moving to the left or "-90" when moving to the right.
The more universal solution where enemies can walk around any solid object would require more efforts. You can make your enemy to walk around sprites (demo, capx) . But you'll also need some way to determine where the enemy should stop and turn around or descend to the floor. Maybe again put an invisible sprite, on collision with which the enemy will change direction or descend.