I'll try the base out now, and upload a pic ASAP :)
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They don't need to target the mortar - as long as the defenses target them first, the WB's can waltz in and blow a hole through to your vault.
I use this on my base, mainly because I found that the outer buildings cause the deployed troops to bunch together after the building is destroyed, making them easy targets when they enter splash damage range.
Where my layout differs from this one is that I keep my outer buildings within range of my turret defenses, which is not the case here. The east and north sides are particularly vulnerable to an archer clear of the outer layer with minimal troop deployment.