Creating terrain and dioramas (camps) for skirmish games is easy -- each figure represents a single combatant. The terrain is whatever would influence the movement or combat of a single combatant. So everything is the same scale -- if your figures are 28mm, your trees, brush, rivers, tents, should all be the same scale.
Games like Triumph! simulate a field battle. A stand of troops represents a very large number of such guys -- usually many hundreds, up to a thousand. The figures on the stand mark what type of troops they are (Elite Foot, Knights, Archers, whatever) but each single figure does not represent a single fighter. Similarly, the terrain represents a whole patch of forest (hundreds or thousands of trees), not three or five of them. And the camp represents the logistical element of the whole army -- thousands of tents, non-combatants, wagons, and so on.
When making camps, therefore, we have some interesting choices. We can build a diorama out of figures that are in scale for the figures of the army, or we can go down a scale. Going down a scale allows us to create "larger" dioramas for camps without having the physical dimensions of the camp create problems for fighting the battle. This sort of forced perspective looks fine on the table.
In the same vein, using 6mm scale buildings for villages and 6mm scale trees as scatter terrain for woods works fine on the table, but that is a topic for another time.
When your camp is built of figures that are the same scale as the rest of the army, your options for diorama topic are focused on a single topic, with a few figures telling a story. A last stand of praying monks on a hill; a small group of warriors, a gate in a palisade, repairing a chariot with a broken wheel, a pack train.
The figures are usually available as leftovers from building the army.
Forced Perspective is when things in the background are deliberately made smaller to make them seem farther away. It works great for dioramas and camps as well. I often use 6mm scale (1/285th or 1/300th) figures and terrain for camps where the main army is 15mm figure scale (1/100th, essentially).
Using a lower figure scale allows you to create a diorama of a larger image -- rather than being focused on a single small image, you can see a larger sweep of things. Without creating a camp whose physical dimensions make it difficult to actually fight battles.
Some camps are scale independent -- they can be used for armies of multiple different scales.
With these types of camps, the main determinant of whether it works with an army becomes the size of the camp -- too small looks stupid on the mapboard; too large looks silly in another way (and has a negative impact upon play, reducing the deployment zone). In most cases if the camp is too big, you can just slide it back (off the playing mat) so less of it is on the map. As long as your table surface permits.
Scale-independent camps don't have figures on them.