Just want to put this out here:
I think minefields would be a great addition to the game (not just as a GP improvement though, workers or specialized units should be able to build them normally.
It just doesn't make any sense to have them invisible though. Sure, on a tactical scale, a lowly infantryman may not see a mine he's about to step on, but a minefield is not something that can really be hidden. Whenever Monty attacked Rommel at El Alamein, he didn't just stumble in and go 'Hey! There's some mines here!'. At Gallipoli the British Fleet weren't oblivious to the fact that there were mines guarding the straight.
Mines should not be invisible to any party.