A linear obstacle is a piece of terrain less than 1" in depth, usually a low wall. Linear obstacles give units behind and within 1" of it +2 DEF against melee attacks, and +4 DEF versus ranged attacks.
Look at it this way, terrain is there to provide advantages to units, and if you alternate placement, it's part of the game to set up the terrain so that you have tactical advantages for your units. Your opponent has just as much ability to set up terrain as you do, so there's no unfair advantage to gaming terrain placement. Just so long as it doesn't get out of hand, in which case you can have certain gentlemen's agreements.
Another important advantage of linear terrain, is that it can easily block charge attacks. A unit cannot charge over a linear obstacle unless it has pathfinder, and no unit can charge over a linear obstacle if there is not room for the model after the charge. So if you place your units within 1" of the obstacle, and "fill up" the wall, there's no room for charging units, even if they have pathfinder, to complete that charge movement.
Also, there are counters. Reach attacks allow units to reach over that wall, and spray attacks ignore cover. And they can always walk around the wall. It's simply an advantage that will probably give your wall hugging troops a round or two of terrain advantage.
I picked up a bottle of Loctite Ultra Gel yesterday. The problem with typical super glues is that they are brittle, and they also do not resist shearing forces very well. Super glue can give a strong bond, but not a flexible one. I recently found out that some superglue formulas include some rubber into the cyanoacrylite, giving it the ability to resist those sheering forces, and some flexibility so that it doesn't *snap* so badly. I hear good things about the performance of these types of superglue, and am going to give it a try on my next batch of minis.
