Keep it simple, stupid.
That's it. I wanted to make a very very simple game, one that you could potentially play (at least in essence) with bits of paper. What I came up with was a grid based game placing importance on the placement of pieces and the economy of use. It started with the premise of there being x number of enemy units on a grid and the player having to position a limited number of templates to fit over them all in one go.
There where three types of template. Each template when used would take two turns before they could be used again, the focus was not just on thoughtful placement, but also use. Planning ahead to decide when you would need a certain shape. I bolstered this out with a combo mechanic. To get maximum effect each template should be placed in the row/column as another. This added another dimension of choice. To place templates for maximum coverage or maximum effect.
I didn't plan a turn based strategy when I started. The game was mostly based around finding an alternative way of approaching a Match-3 concept of making changes on a fixed grid with a focus on placement. Something that players of Bejewelled and such can identify with and yet find different and refreshing from playing gem matching games.