Got any tips on creating a good scenario? Triggers, eyecandy, etc.

I am currently planning on creating a custom campaign (first time), and I need all the tips i can get.

Two problems specifically that I'm facing, if anybody can help... (1) trying to play the scenario from the main menu doesn't work. (2) Is it possible to trigger a victory by defeating a town center? It seems that my trigger is on the settlement, not the town center, so I can't destroy it.