After Effects Set Extension

To create this set extention motion tracking needs to be set from the vert start of the clip after this select track motion and allow track rotation. Select the points and put them each on a significant part of the composition for example a leaf or a rock. Anaylse the comp until point 1 is about to come out of the comp, from here I created two null objects naming one first and the other second. now the first point will track the first null object so the motion tracker can now anaylse more of the comp.
Select track motion again so two new points are created and find two building in the background and put each point on them, anaysle the compostion again. apply this motion target to the second null, and then parent the second null to the first.
From here in Photo shop the castle can be edited and placed onto the background we have copied so the castle now looks like it is part of the shot and can add more in the distance and change it colour around it to match where the man is walking. Put this into the comp making it's own composition so that the clip all goes as one creating the new background.