  We study a variation of the cops and robber game characterising treewidth,
where in each play at most q cops can be placed in order to catch the robber,
where q is a parameter of the game. We prove that if k cops have a winning
strategy in this game, then k cops have a monotone winning strategy. As a
corollary we obtain a new characterisation of bounded depth treewidth, and we
give a positive answer to an open question by Fluck, Seppelt and Spitzer
(2024), thus showing that graph classes of bounded depth treewidth are
homomorphism distinguishing closed. Our proof of monotonicity substantially
reorganises a winning strategy by first transforming it into a
pre-decomposition, which is inspired by decompositions of matroids, and then
applying an intricate breadth-first "cleaning up" procedure along the
pre-decomposition (which may temporarily lose the property of representing a
strategy), in order to achieve monotonicity while controlling the number of cop
placements simultaneously across all branches of the decomposition via a vertex
exchange argument. We believe this can be useful in future research.
