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.