Each level of a tiered cake needs dowel rods for support. Tiers that are directly stacked on each other with no separation by plates and pillars should be further stabilized with an additional center dowel rod to prevent shifting. With a pencil sharpener, sharpen one end of a long, wooden dowel rod slightly shorter than height of two tiers. Push sharpened dowel through top tier to cake circle, then, hitting sharply with tack hammer, drive dowel through cake circle to bottom of cake. If there are three or more tiers without separation by plates and pillars, use a sharpened dowel through the first two tiers and another sharpened dowel through second and third tiers.
*An Exception--Using Plates to Separate Stacked Tiers The type of cake sometimes dictates what is placed between the layers for support. Some decorators prefer using separator plates between stacked tiers instead of cake circles when the tiers are heavier than usual (such as fruit cake or dense chocolate cake). This prevents the possibility of the cake circles warping from the excess moisture and weight, thus causing the layers to shift. Of course, if you use separator plates instead of cake circles between the stacked tiers you will not be able to drive a single dowel rod through the stacked tiers for the extra reinforcement as above.