Base building mechanics, possibly grid based - Help!

We’re currently working on a project in which you can set up traps and build certain things in the environment. Nothing as advanced or complex as Fortnite for example, but more along the lines of the traps in Sunset Overdrive. Certain things can be placed anywhere and certain things like doors are obviously defined slots. I think we want to go for something based on a grid rather than complete free placement. So I’m just wondering where to start to figure out all of this. The things I need to accomplish are:

Have a top down view in which players can move around the player space (easy enough)
Hover over build locations and highlight them (Should be easy enough)
Any trap that is built needs to be swapped with the active version

Traps that are ‘freely’ place-able need to to be on a grid.

Does anyone have any advice on where to start and how to go about doing the grid or if there is a better method? Thanks!