Now there are, in my experience, usually ways that the player facing the 'camper' can deal with this situation. Armies that have good skirmisher and medium options can surround and erode away at the defensive position until it becomes vulnerable to an assault. Even heavy units can eventually get the upper hand sometimes if they can keep manoeuvring around the stationary army and get enough force concentration and flank attacks in at a particularly exposed area. (Cavalry can be surprisingly useful in mountain assaults as they can flank, disrupt, drop back, flank and fragment, and then break the enemy without getting sucked into melee on the bad ground. Don't discount your cavalry in such a situation.)
However, my main topic for this post is more from the camper, or should I say, the defender's point of view. There is passive defence, which is simply holing up on difficult terrain to assault and hoping you can repel all attacks by the time the sun goes down, and then there is the (usually much more effective and viable) active defence.
A player who is practicing active defence may start in a very defensible position; on top of a mountain, behind a marsh or stream, between rough ground on one side and a built up area on the other, etc. The expectation is that the attacker will start to move up and encircle or chip away at the position over time. However, most of the time the attacker will have to split up their force a little bit or focus on a particular part of the line in order to achieve this. They will also have to invest resources in winning the skirmish. A player who is practicing active defence will wait until the attacker is at their most vulnerable point (usually just before their flanking or breakthrough forces are actually in position for an assault) and then strike forth from their defensive position against the most exposed part of the attackers line; perhaps one that has been worn down or disrupted by arrow fire, or just a part of the line the attacker has left thin.
Experienced players on the offensive against a tough position will, of course, manoeuvre in such a way as to minimise these chances, and will keep the defenders bottled up under missile fire if they can win the skirmish. However, even the most experienced players won't be able to keep all avenues of attack closed to the defender all of the time if the defender has staged his or her troops properly. This is how armies that seem like they don't stand a ghost of a chance out in the open (perhaps due to low frontage, like the expensive Avars, or due to squishiness, like a massed archer army) can get the upper hand against dangerous heavy or medium opponents without giving up all initiative and drawing out the battle.
This can also be a way to wriggle your way out of a draw if both armies are unwilling to advance. If the heavy or the medium army exposes just a bit of themselves, either by the heavy army making some inroads into the difficult terrain, or by the mediums drawing up part of their army out in the open, it can lure the other army in and start the battle on more reasonable terms. Which side gets the advantage still comes down to who can manoeuvre best and pressure their opponent into making the most disadvantageous moves... another reason to take your skirmishers with you if a stalemate seems likely!
Though this does underline a need to be a bit flexible. Persian Sparabara can't fight out in the open against Greek hoplites very well, so having part of your line anchored by forest or rough ground isn't an unreasonable situation. But if your entire Persian army is in dense forest, there really isn't much the Greeks can do, just as an example.
(And believe me, I've tried. My heavy infantry assault on a forest full of bowmen did not end well.
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