I've found that Watchtower isn't an engine card so much as a terrific utility card — it's a bit of Moat, Library, Royal Seal, and, um, Make Goons Even Awesomer rolled into one.
I mean, Festival/Watchtower is pretty sweet, but Festival/Library is much better. And in this game, Throne Room/Pawn/Watchtower was neat, but KC/KC/Rabble just blew you out of the water.
But I do like Watchtower, precisely because it's so flexible; it's quite common that I use it in three different ways in the same game. Which is fun. It's just not something I see and immediately start planning around.