I was thinking about this today. That would be okay, but I think the better option (or both would be fine, they don't have to be excludive) would be the ability to bring up the upgrade page from anywhere on the map, just to see what upgrades have been purchased. I'm not saying you could upgrade them from anywhere, but this way you could say to yourself "Did we get catapults yet?" and just click a button and open up the upgrade menu to see if you should make the trip back.