This is what we are talking about with formation exploit:
formation capture exploit.mp4
And this:
formation gathering exploit.mp4
The problem with this approach is that, if you need to garrison, say, around 40 units to prevent your building from being captured, out of an average of 150 soldier units you might have in a 200 pop game, you end up dedicating 26% of your units just to guarding a single building. It makes more sense to increase capture resistance.