It’s easier to build territory in the corner, or against the sides. However, it is also possible to build territory without any help at all, despite your opponent’s best efforts to stop you.
This example assumes that neither side has any preexisting posts that can help. So this invasion would work inside Blue’s territory as long as the territory was large enough that none of the Blue boundary posts could help.
Red starts with 1, a lone post in the wilderness. Blue can respond in several ways, but none will work. 2 is as good as any. Red 3 establishes a fence, the key property of which is that it can form two different square diamonds, neither of which contains Blue 2.
At this point Blue has a choice of which square diamond of Red to block. Blue chooses 4, blocking Red from playing down and to the right. Red responds with 5, forming two sides of a square diamond and threatening to make territory with his next move (at 6).
Blue could also play 4 at 5 or 6, in which case Red would play at A and connect to 1, with 3-1-A serving the same purpose as 3-1-5 does.
If Blue plays 6 to block the square diamond then Red seals the deal with 7. This move threatens two different territories: the long diamond at B, and the thin diamond at C. Blue can block only one of them, so the other will give Red territory.
This sequence assumes Blue is foolish enough to try to stop Red. He can’t, so a better strategy would be to try to minimize Red rather than stop him altogether. The correct strategy would depend on the layout of the surrounding posts.