The game includes 10 generic scenarios like you describe. Meeting engagement, hold the gap, river crossing etc. So you can choose one randomly.
There is a "pick up" terrain generation system. You bring terrain suitable for your army (desert for Arabs for example), sufficient to fill 25% of the squares on your table.
You get to place one piece of terrain per 8 squares on the table. There are limits to how much of each type may be placed. Players alternate placement, with a restriction for right, center, left.
Once complete a card is drawn (the game uses cards, not dice) for each piece. Based on the suit/value of the card drawn the terrain piece may be left in place, moved, or removed entirely.
A nice wrinkle is that "open" is a terrain choice. So you can place an open terrain in a box to block your enemy from putting a woods in it – handy if you have lots of phalanxes, for example.
Rules also specify how close terrain can be, etc.
