Back to basics -- stone on stone. Anchor Blocks from Anker Steinbaukasten GmbH, Rudolstadt in Thuringia.
Outdoors on a sunny afternoon, with a breeze not quite strong enough to keep me from smelling the wonderful Ankerstein aroma. Linseed oil for sure ... chalk probably ... can one smell quartz?
I built this just outside our sliding glass back door, which I had left ajar a crack to ventilate the house a bit. Archie could hardly stand it, lying on his side and extending his paws as far as they could reach. Webster was calmer, though I suspect him of even greater hidden frustration. But they are indoor cats, and today was not a day for cats in harnesses, on leashes. Somehow I do not think that would mix well with block play.
It was good block play.
[Addendum -- I ran across this image of the Bremerhaven train station, and thought how my exposure to German architecture contributes to my appreciation of Ankerstein. One need not have gone so far afield -- many older American brewery structures are classics of the genre.]