Are my walls too thin to keep creepers from arming?

Question

Some of my friends and I are building a wall to encompass the entirety of our house and farms, but creepers tend to explode and flood us with zombies in the night, because the walls are made of cobblestone fence (cobblestone wall). I am wondering if they are blowing up because cobblestone walls are slightly thinner than regular blocks (I believe they are still thick enough to prevent coming within 'arming' range of creepers, but I may be wrong), or is it because we are standing on top of the walls and the creepers jump up to us?

Question: Can creepers get close enough to "arm" (start hissing), even if they are on the other side of Cobblestone Wall blocks? The reason this might be possible is because doors and glass panes are too thin, but cobblestone walls might be thick enough.

W = CobbleStone Wall
I = Iron Bars
H = Wooden Doors
O = Log

FRONT VIEW - DOOR SECTION

I I I I I I I I I I I I I I I I I I I
O W W W O W H W O
O W W W O W H W O

The walls are 2 Cobblestone Walls high, but then have Iron Bars on top of them. Note the door, there is a slight gap between the door and the wall, so creepers may be sensing us through this gap

Answer 1

Creepers can certainly get close enough to arm through cobblestone wall, but that's not the problem; they can even get close enough through a two-layer thick wall of obsidian; their arming range is a full three blocks. And yet, even a door is usually thick enough to stop them.

Creeper arming is not just based on distance, but also visibility. Cobblestone Walls are a block and a half tall for collision purposes, but not for Creeper line-of-sight. If the walls are only one layer tall, creepers have a clear line of sight over it and can still arm and fire.

Note that even transparent blocks like glass, glass panes, etc. block creepers' arming. And they can't see through the holes that form in Cobblestone walls more than one layer high; the blocks all still count as completely solid. However, the blocks aren't completely solid when they run up to most other blocks. So, while a glass wall,

a cobblestone wall,

and doors

are all fully able to stop them from arming, placing either glass or a door next to the walls gives them a gap to see you through:

Additionally, you can stop them from arming by getting much higher ground on them. If the top of their head is at least a full block below your feet, they won't arm.