Draw until you have 6 cards in hand.
When you gain a card, you may reveal this from your hand. If you do, either trash that card, or put it on top of your deck.
When you play this, you draw cards one at a time until you have 6 cards in hand. If you have 6 or more cards in hand already, you do not draw any cards. When you gain a card, even on someone else's turn, you may reveal Watchtower from your hand, to either trash the gained card or put it on top of your deck. You may reveal Watchtower each time you gain a card; for example if another player plays Mountebank, you may use Watchtower to trash both the Curse and Copper, or to trash the Curse and put the Copper on top of your deck, or just to trash the Curse, and so on. You still did gain whatever card you gained; you just immediately trash it. So if Mountebank gives you a Curse and you trash it, the Curse pile will go down by one as the Curse gets moved to the trash pile. You may reveal Watchtower on your own turn as well, for example when buying a card, or gaining a card via something like Expand. If you use Watchtower to put a card on your deck but have no cards left in your deck, you do not shuffle; the gained card becomes the only card in your deck. Revealing Watchtower does not take it out of your hand; you could reveal Watchtower on multiple opponents' turns and still have it on your turn to draw up to 6 with. When cards are gained during a Possession turn (from Dominion: Alchemy), the player who played Possession is the one who can use Watchtower, not the player who is being possessed. If a gained card is going somewhere other than to your discard pile, such as a card gained with Mine (from Dominion), you can still use Watchtower to trash it or put it on your deck.