A fluid flow plate for an electrochemical assembly, defining an x-y plane and comprising: a plurality of first fluid flow channels (13, 14, 22) extending across a face of the flow plate to define a first fluid flow field of the fluid flow plate; a plurality of second fluid flow channels (13, 14, 22) extending across a face of the flow plate to define a second fluid flow field of the fluid flow plate, the pluralities of first fluid flow channels and second fluid flow channels both occupying a common channel level defining a finite space along the z-direction; an array of first fluid transfer points (301, 302, 303) disposed along an edge of the first fluid flow field for communicating fluid into or out of the first fluid flow channels (13, 14, 22); an array of second fluid transfer points (301, 302, 303) disposed along an edge of the second fluid flow field for communicating fluid into or out of the second fluid flow channels (13, 14, 22); a first fluid distribution gallery (21, 15, 16) having a first peripheral edge portion bounded by the array of first fluid transfer points (310, 302, 303) and having a second peripheral edge portion forming a first fluid communication edge (31, 32, 34) of the fluid flow plate, the first fluid distribution gallery comprising an open array of projections and configured for fluid communication and distribution between the array of first fluid transfer points across a full width of the first fluid flow field and the first fluid communication edge; a second fluid distribution gallery (21, 15, 16) having a first peripheral edge portion bounded by the array of second fluid transfer points (310, 302, 303) and having a second peripheral edge portion forming a second fluid communication edge (31, 32, 34) of the fluid flow plate, the second fluid distribution gallery comprising an open array of projections and configured for fluid communication and distribution between the array of second fluid transfer points across a full width of the second fluid flow field and the second fluid communication edge; wherein the first fluid gallery occupies a first gallery plane and the second fluid gallery occupies a second gallery plane different from the first gallery plane, and in which both the first gallery plane and the second gallery plane are disposed within the common channel level; characterized in that at least one of the first and second fluid communication edges (31, 32, 34) comprise a castellated structure. a plurality of first fluid flow channels (13, 14, 22) extending across a face of the flow plate to define a first fluid flow field of the fluid flow plate; a plurality of second fluid flow channels (13, 14, 22) extending across a face of the flow plate to define a second fluid flow field of the fluid flow plate, the pluralities of first fluid flow channels and second fluid flow channels both occupying a common channel level defining a finite space along the z-direction; an array of first fluid transfer points (301, 302, 303) disposed along an edge of the first fluid flow field for communicating fluid into or out of the first fluid flow channels (13, 14, 22); an array of second fluid transfer points (301, 302, 303) disposed along an edge of the second fluid flow field for communicating fluid into or out of the second fluid flow channels (13, 14, 22); a first fluid distribution gallery (21, 15, 16) having a first peripheral edge portion bounded by the array of first fluid transfer points (310, 302, 303) and having a second peripheral edge portion forming a first fluid communication edge (31, 32, 34) of the fluid flow plate, the first fluid distribution gallery comprising an open array of projections and configured for fluid communication and distribution between the array of first fluid transfer points across a full width of the first fluid flow field and the first fluid communication edge; a second fluid distribution gallery (21, 15, 16) having a first peripheral edge portion bounded by the array of second fluid transfer points (310, 302, 303) and having a second peripheral edge portion forming a second fluid communication edge (31, 32, 34) of the fluid flow plate, the second fluid distribution gallery comprising an open array of projections and configured for fluid communication and distribution between the array of second fluid transfer points across a full width of the second fluid flow field and the second fluid communication edge; wherein the first fluid gallery occupies a first gallery plane and the second fluid gallery occupies a second gallery plane different from the first gallery plane, and in which both the first gallery plane and the second gallery plane are disposed within the common channel level; characterized in that at least one of the first and second fluid communication edges (31, 32, 34) comprise a castellated structure.