General game architecture tips?

[south tangle]

Hi guys, i see a lot of tutorials on specific subjects.

And in general i have no problems with these, but i run into problems later on when i try to move on to bigger projects

For example, here, i am trying to make a tower defense game based on a grid system

I'd like to be able to select a square and based on a lot of different things (type of terrain of that square, player money, if it is already built => upgrade the building, depending on what the player has unlocked and so on), i tent to get lost into a lot of spaghetti code and i dont know how to structure all that