How high does Entangle go?

[Timrod]

I've got a seemingly simple and straightforward question that I'm not sure is written anywhere in the rules, but I want to make sure.

I mostly run PFS games, but this would apply just as readily to a home campaign if something similar were to come up. The scenario I have is as follows:

An enemy starts their round standing on a wooden tower that is 20 feet tall. The tent is (logically) situated on ground that is legal for entangle. The enemy casts entangle in such a way that it affects the squares underneath them.

Logic dictates that there SHOULDN'T be any way that the enemy is affected by the entangle spell, because they are standing on an artificial structure with no mention of there being plants climbing up it.

I know with Web that it's a 20-foot radius, meaning it extends out 20 feet in all directions and is also 20 feet high. For Entangle to do the same thing, it would mean seeing grasses and shrubs reaching 20 feet up to grab the person on top of the structure. As a GM in a home game, I would definitely rule that the enemy is not affected (but anyone at the bottom of the structure would be).

Are there any written rules on how this works?

[Calth]

Entangle does cause plants to grow in anyway, so if there are no plants on the tower the people on the tower are not entangled. For reference, mythic entangle does cause new growth, and that is limited to 10 ft in reach, so 20 ft clears even mythic entangle.

[Diego Rossi]

Entangle wrote:

Area plants in a 40-ft.-radius spread

It affect the existing plants. No plants, no entangle.

On the other hand: "An enemy starts their round standing on a wooden tower that is 20 feet tall. The tent is (logically) situated on ground that is legal for entangle. The enemy casts entangle in such a way that it affects the squares underneath them."
The enemy has a wooden floor under his feet?
If so he can't target the square under him. He should target a square he see.
As the AoE is a spread, this apply:
"A spread spell extends out like a burst but can turn corners. You select the point of origin, and the spell spreads out a given distance in all directions. Figure the area the spell effect fills by taking into account any turns the spell effect takes."
so probably the AoE will be somewhat deformed by the presence of blocking terrain, like a tower walls.