This small diorama was on display at Warhammer World. It features mounted Chaos Warriors attempting to attack a bastion of the Empire.
It could be an entrance to a castle, or perhaps a city wall.
It’s not a diorama I recollect seeing before at Warhammer World or in a copy of White Dwarf (then again I don’t buy every copy of White Dwarf). It was in the first section of the exhibition, which makes me think it’s old.