Dominion: Prosperity
At $3, Watchtower is the most expensive Reaction thus far, but well worth the price. It is one of the most versatile cards in the game, and the only Action thus far that provides a benefit even without playing it.
First, and most obviously, Watchtower is a devastating defense card against Curse-givers. Not only does it block the Curse from entering your deck, it actually trashes the Curse, meaning that an attack-heavy opponent will soon find himself running out of Curses to give. (Compare to Moat, which, against a Witch-heavy player, only delays the inevitable.) Although it provides no defense against “deck-inspection” attacks (e.g. Spy, Thief, Pirate Ship, Rabble), it does subtly defend against handsize-reduction attacks by allowing you to draw back up to 6.
Second, if you’re willing to hold onto Watchtower and not play it, it works as a mini-Royal Seal, minus the $2. There are some ridiculous ways to take advantage of this; most powerful is probably playing a Talisman, buying a Treasure Map, gaining another one, and putting them both on the deck! More commonly, you can buy something like Village/Smithy or Throne Room/[some other action] and ensure that you draw them together next turn. Even when you just have one buy, Watchtower can make sure you play important attack cards before your opponent gets a chance to.
Third, and this is a very specialized use, Watchtower greatly increases the power of Goons. Ordinarily, Goons-heavy players will avoid buying too many Coppers for VP tokens for fear of weighing down their deck. Watchtower solves this problem by trashing the Coppers as they come in, while still collecting the VP tokens.
Finally, Watchtower can form part of a good card-drawing engine. Although Library is superior (both because it draws more cards, and also because it can discard unwanted Actions), Watchtower is much cheaper and fulfills other roles as well. In addition, drawing a dead Watchtower (either because you have no Actions left, or because your hand size is very large already) is much less of a liability than a dead Library because of its other uses, as noted above. It is especially useful with Vault.
Works with:
- Festival, University, and other +Actions that do not replenish your hand
- Goons
- Vault
- Talisman, or other sources of multiple Buys
- Curse-giving attacks (Witch, Mountebank, Torturer)
- Handsize-reducing attacks (Militia, Goons, Torturer, Ghost Ship)
Conflicts with:
- Cards that increase hand size (e.g., Laboratory/Alchemist, Caravan/Wharf, Tactician)
- Attacks that it does not defend against (Pirate Ship being the most important)
- Royal Seal, since the benefit becomes a bit redundant
- Library, since Watchtower is an inferior engine card
Dominion Strategy 
Also combos well with Ironworks/Workshop, as you can gain a card costing up to $4 and put it on top of your deck. If you have enough actions, you can then use the Watchtower’s drawing ability to put the card into your hand.
Once I Throne Roomed an Ironworks, gained two Treasure Maps (giving me +2 actions), showed the Watchtower from my hand, put the Treasure Maps on top of my draw pile, played the Watchtower to draw the Treasure Maps, then trashed the two Treasure Maps for four Gold, all in a single turn. That was fun.
The Watchtower, like the Scout, is a card that makes other cards better. It benefits the strategic player because it’s so versatile. It’s also an effective reaction card. This is one of my favorite cost 3 cards.
I forgot to mention that this works even better with University, since University provides two extra actions. One could, for example, play the University, gain a Witch, show the Watchtower, put the Witch atop his deck, draw the Witch with the Watchtower, then play it. This is perhaps the easiest way to gain an action card and play it in the same turn.
The watchtower, unlike the scout, is actually good in many situations ;).
I especially like watchtower/ironworks combos when there is an island or great hall in the deck. You can set off a bunch of iron works, gaining cards putting cards on top of your deck, knowing that you can draw them to play them on the same turn.
On turn 10 here, I use the watchtower/ironworks/cheap mixed action vp combination to play 5 iron works and a island on a single turn without any really strong cards in the deck. Nothing in my deck cost more than 4.
http://councilroom.com/game?game_id=game-20101125-212646-1e318008.html
The text needs a bit editing in the beginning since Horse Traders is now the most expensive Reaction card.
There is now Trader in Hinterlands, which also costs 4.
When combining Watchtower with Goons, you could use your extra buys to pick up Curses to trash. This would be a nice preemptive strike against curse attacks.
Watchtower provides a nice little combo when buying cache, as you can put it on the top of your deck while trashing the coppers that are the price of doing business with that card. In fact, most cards-at-a-bad-card-price side effects go away with Watchtower.
This is true but just to play devil’s advocate, you have to keep in mind that if that Watchtower had been practically *any* other useful card you could have afforded to buy a Gold instead of Cache anyhow. Likewise, you probably could have bought the Gold if you’d played the Watchtower for its Action ability. So all you’re really gaining from the Watchtower trick is that Royal Seal effect.
To put it another way: usually Watchtower acts like a crappy Royal Seal that gives $0 instead of $2, and is redeemed by its low cost and alternate uses. In this case it’s acting like a crappy Royal Seal that gives $1 instead of $2.
Well, I wouldn’t suggest buying Watchtower for its synergy with royal seal, but if you want one anyway, this is another bonus for it.
Watchtower/Cache is pretty nice, too. $5 Gold with no drawback! I had a Feast, Watchtower, and $5 in my hand one turn and got two Caches on my deck for my next turn.
I’ve decided that I don’t think Watchtower conflicts with anything. The top-decking/trashing ability is far superior to any lost potential card-in-hands. In fact, I’d almost never use the drawing ability unless I had 1) multiple WTs in hand, or 2) was intending on buying Victory cards.
The bigger your deck gets, the stronger that top-decking ability becomes. I just won a Vineyard game where I didn’t buy a Potion until almost midway through. Then I bought 2 Potions and a Hamlet, putting them all on top of my deck, guaranteeing myself 2 Vineyards the next turn.
In addition to some of the other combinations, if there are no really good $5 cards out there, you could use the WT to buy an Ill-Gotten Gains and trash it immediately, in effect spending $5 to give your opponent(s) a Curse.
If I drew Estate, Estate, Estate, Market Square, Watchtower, could I:
1. Skip actions,
2. Buy Copper for $0,
3. Show Watchtower in reaction to gain Copper, deciding to trash,
4. (the actual question) Is such trashed Copper “mine” enough in order to have Market Square reaction trigger and turn it into a Gold?
yes
Actually, if you drew Watchtower with Market Square and anything less than $5-6, the best move might still be to buy any card and trash it for the Gold. Doesn’t need to be this hypothetical $0 situation. And if Fortress is available at $4? Free Gold!
It’d be nice if this article were updated to include some of the later expansions, especially Dark Ages, as some of those cards have a “when trashed” effect.