It's a tower, not a wall. Same as the one Sakuya is holding on the first page.
Explanation courtesy of glasnost:
The joke is kimashita waa vs. kimashi tawaa. "Kimashita waa" would be the polite version of "kitaaaa" and is used when you are excited by yuri. But it sounds like kimashi tawaa, or kimashi tower.
An English analogy would be something like Koakuma constructing a ship (and Sakuya also holding a model ship on the first page).
The sign might say "kimashi tower planned construction site", but he can't be certain on mobile.
EndIt's better if you don't build that.Koakuma? Look.fufu
If you're so inclined, shall we go ahead and do SakuPatchy?That would be a most wonderful invitation, but it is a Koakuma union this time, no...?N-no, I just...I had never entertained the notion that Patchouli-sama would publicly display her affections, you see.Is this a good place for the Kimashi Tower?There's even cake, too?Huh?I'LL GO GET ANOTHER CUUUUUUUUP!!!How about we have some tea soon?So why have you frozen up?(Vocal Crying)I-Ah, what a shame.