arahkin: When you get to the point where 3 doors block the core 3 red lights will show up.
singbird: In attach1 is the initial situation after hitting the core after only one lightbulb has shown up. There are three doors. In attach2 I have lit both the two bulbs and the left and the right door have opened, but there's still the center door. What do I have to do in this situation?
From what you say it sounds like there should only be two doors, though.
Well from what I can tell based on those images you have encountered a bug. That middle door is suppose to go away after the first core hit but for some reason has come back out. If exiting the game and reloading doesn't work I would suggest contacting the makers through their website
http://sumomgames.com/ or try reinstalling the game to see if that works. I'm also assuming you have tried shooting through that middle door after getting the other 2 open? If not try that to see if it is just a small graphic error. But yea that door is not suppose to be there like that.
Sorry but that's about all the help I can offer on this. Hope you are able to get it fixed.