The second half of judging wilderness encounters is handling evasion and pursuit. The rules don't come out and define the difference, but evasion is the chance of remaining undetected by the other side, while pursuit is avoiding capture by the other side. The other side can't pursue if the party successfully evades; they simply don't realize the party is there, or lost track of them after leaving the castle.
A typical party can evade monsters or castle occupants by making an Avoid Accident roll. This assumes that the maximum number of creatures possible is out patrolling the area; add +1 to difficulty if there are less than 2/3rds of the maximum number of monsters, or by +2 if there are less than 1/3rd. (I'm not certain what the rationale behind this is. Are the monsters bumping into each other or distracting each other?)
If the party isn't a normal size, the odds change. A party of 3 or less reduces the roll by -1, while parties of 10 or more find evasion more difficult: +1 to the roll, or +2 for 25 or more. The odds also change if the terrain is woods or something similar: reduce the roll by -2 difficulty because of the extra concealment.
If the party surprises the monsters or castle patrol, they reduce the difficulty of the evasion roll by -3, double any other reductions and halve any difficulties mentioned above. Thus, a party of 25 has only a +1 difficulty instead of +2, and surprising monsters in the woods reduces the roll by -4 instead of -2.
On the other hand, if the party is surprised, there is no chance of evasion unless the encounter is in the woods; in that case, make an Avoid Danger roll instead of Avoid Accident and disregard the above modifiers.
If the party is twice as fast as the other side, reduce difficulty of either the Avoid Accident or Avoid Danger roll by -2; if the party is half as fast as the other side, increase the difficulty by +2.
Pursuit is much simpler: to avoid capture, make an Avoid Accident roll at +1 difficulty in most terrains, -1 difficulty if in woods or swamp. This only applies to a party being pursued by faster creatures; if the speed of both sides are the same, capture is impossible, but pursuit continues until something changes (party uses magic, chase moves into terrain only one side can cross, a third group interrupts the chase, one side becomes exhausted.)