You start out by defining your character by giving it:
Skills / Abilities / Perks / Quirks
Optimal biography which might have slight impact in game as you progress
Decide your characters color or ASCII tile; this being the default character you will use ()
Then you decide if you want to defend or attack the gates.
There is a big difference between defending and attacking the gates.
You have a strip of land that you will defend, as seen on the right side.
You get to play around with several levels of it, that is z(the level you are looking at now), z+n(the space over z) and z-n(underground of z).
You may alter these levels for your needs, either by digging and channeling or by placing walls and such in the field.
You buy monsters to attack the land.
You can decide weather you want or not to enter the field, if you don't you may only interfere what happens once a wave. If you do enter, normal interaction rules applies.
If you decide to start defending you may choose an archetype.
If you decide to start attacking you you have a starting pool of money, but you don't get any rewards other than the destruction and mayhem you cause. You may also decide if you want to begin in the middle strip(default), or in the left or right strip. For those who don't understand the difference I won't spoil the "FUN".