So, buying a Rocks with Watchtower in hand can end up with two Silvers on top of the deck, right? Or not?
Right. When you gain Rocks, it triggers the on-gain on Rocks and also the Watchtower reaction. You decide what order to resolve them in. No matter what, you're going to gain the Silver from Rocks. If you also use Watchtower to trash the Rocks, it will trigger the Rocks' on-trash and gain you a second Silver on top of your deck. You can also use Watchtower to trash either of those Silvers, but that's usually a bad idea. You probably don't want to trash the Rocks with Watchtower either, since it's also good Catapult fodder. If you weren't after fodder, you probably shouldn't be buying Rocks (edge cases: emptying the pile, Fairgrounds).
Oh man. Fairgrounds gets another buff from split piles. Did somebody mention that when the expansion was announced? I don't remember, but this is a thing.
Anything that combos with Feodum, combos with Catapult. Trash a Silver with your Catapult, and each opponent gets to discard AND take a purple. Too bad a hand with Catapult + 4x Silver doesn't get to Province...
Well, Catapult + 4x Silver
does get to Province. Just don't play the Catapult. Great point about Feodum; it's another great source of Catapult fodder. Why throw rocks when you can throw an entirely piece of property?