The bulk of the voltage multiplication in this circuit comes from the diode/capacitor ladder circuit; not the transformer.
If a transformer winding is rated for voltage use (as in a power transformer), then a center tap is used when needing to drive a primary with half the voltage. e.g. a power transformer can have a rating like 120V/240V, meaning that if you want to drive it with 120V, you would connect power to 1-2 with 3 floating. If you want to drive it with 240V, you would connect power to 1-3 with 2 floating.
If a transformer is rated for impedance (as in an audio transformer), then the impedance goes as the square of the turns ratio, so if 2 is a "center-tap", and 1-3 is rated 16 Ohms, then 1-2 or 2-3 would be 4 Ohms. Sometimes on audio transformers, the tap is not centered, in which case you might find something like 1-2 is 3.2 Ohms, 2-3 is ~8 Ohms and 1-3 is ~16 Ohms...
btw-A 'stun-gun' is a very dangerous implement, especially to you. If you threatened me with your homebrew "stun-gun", I would take it away from you, and then proceed to beat the crap out of you...