So one of the basic movement rules says that it costs a creature movement to move INTO a square and not to EXIT a square. Given this rule, difficult terrain is easy to deal with when it comes to medium-sized creatures. The terrain does not affect these when they move out of a square with difficult terrain (or similar) into a square that is not difficult terrain. This movement only costs 5 feet for the creature since no part of it is moving through difficult terrain and no part of it is still in difficult terrain.
Our problem at the table: Imagine a Gargantuan creature, so 4x4 squares, on which Wall of Thorns is cast in circle shape, so 20-foot diameter, basically the same area as the creature, dead center.
To my mind there's different ways to look at this: If the creature moves in any direction, it moves into squares that are not affected by WoT for the 4 squares outside of the AoE. But what about the remaining 12 squares? Do they count as having been moved into the WoT area? Also, at what point is the creature no longer considered to be under the affect of the WoT? Does the rule apply that states that at least half of the creature's base has to be in the AoE? So the creature would be impeded only for the first 2 squares of movement and then be scott-free for the rest?
