[sblock=going over or around the tree]Good question. I think the best mechanical match for this that I can find is the Hop Up action from the Jump skill: "You can jump up onto an object as tall as your waist, such as a table or small boulder, with a DC 10 Jump check. Doing so counts as 10 feet of movement, so if your speed is 30 feet, you could move 20 feet, then hop up onto a counter. You do not need to get a running start to hop up, so the DC is not doubled if you do not get a running start."
If you wanted to remain on top of the tree (and thus gain +1 to attack from higher ground) I'd want a balance check DC 5. Moving off of the tree would just take 5' of normal movement, no check needed. Moving from one tree square to another takes 10' of movement.
Thus, William could move 5', jump up on the tree with a DC 10 jump check and costing 10 feet of movement, move one more square (costing 10') to get adjacent, make a DC 5 balance check to stay on the tree, and attack this round with a +1 to hit from higher ground.
Going around, the dark green squares there are considered forest undergrowth squares, costing 2 squares of movement and providing concealment. So doing that, William would need 30' of movement to reach #1 through the woods, and wouldn't be able to attack this turn.
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(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.