I do wonder how effective all that stone is? How can it stop the town from flooding when the sea level is above it during severe storms (see photo below from earlier this year)? Surely a better solution would be to replace the Prom seating with new seating set into a higher back wall, effectively creating a 3 foot high flood wall along the Prom?
Not sure how much the mean sea level was actually above the prom; I suspect the wind driving the waves was the main culprit in the flood damage.
The main object of the stones is to dissipate the energy of the waves and thus reduce the quantity of water which can breach the prom. If you simply had a wall, there'd be no dissipation at all, merely reflection and absorption. The Llandudno shoreline is part of the coastal interface which is exposed to a wide range of erosional processes and thus a combination of denudational processes will work against a seawall. In purely practical terms, something's gotta give, and if that were to happen in a catastrophic failure then a lot of people could be hurt.
I much prefer the soft engineering option that both Colwyn Bay and Llandudno are currently favouring, although a lot of CB's prom is hard-engineered. I think however, you'd need roughly a million tons of sand to make the thing work really well.
