I was following some basic tutorial on youtube to make basic levels and after I used the solidify modifier to make the walls thicker, this weird glitching happened in the corners. Can you explain to my why and what should I do to avoid it? Thanks.
You need to remove Doubles from your mesh for the solidify modifier to work correctly. Open the Specials menu and choose Remove Doubles.
Then enable Backface Culling and you will see that your mesh is "inside out ". Correct this by selecting all then open the Specials menu again and this time choose Flip Normals.