I recently did a 4 tier topsy turvy cake...and an hour into the event, it collapsed. I used dowel rods on each layer and had all my layers on cake circles. I didn't put a long dowel through the center of all 4 cakes b/c of the cardboard circles. Next time, should I drill a hole in each circle and drive a dowel rod through it? I've make a 3 tier topsy turvy and didn't do it, and it was solid!

No need to drill a hole in the cake boards. Just sharpen the dowel and hammer it in (gently) or use the sps support system and no need for center dowel.

If you use a center dowel, you don't have to drill a hole. Just sharpen the end of the dowel and hammer it down. Works great!

Did you transport it already assembled?
When you placed the upper tiers in place, did you have to move, scoot, or adjust them?
Did it look straight and even when you left it?
Was the table moved or bumped after you left?