(http://wiki.dominionstrategy.com/images/thumb/3/36/Arena.jpg/320px-Arena.jpg)(http://wiki.dominionstrategy.com/images/thumb/5/54/Basilica.jpg/320px-Basilica.jpg)(http://wiki.dominionstrategy.com/index.php/Basilica)(http://wiki.dominionstrategy.com/images/thumb/a/a9/Baths.jpg/320px-Baths.jpg)
(http://wiki.dominionstrategy.com/images/thumb/b/b0/Battlefield.jpg/320px-Battlefield.jpg)(http://wiki.dominionstrategy.com/images/thumb/e/e3/Colonnade.jpg/320px-Colonnade.jpg)(http://wiki.dominionstrategy.com/images/thumb/c/c0/Labyrinth.jpg/320px-Labyrinth.jpg)
(This article is pretty casual and assumes 2 player games.)
All these Landmarks have similar play patterns, so I think it makes sense to have 1 article for all of them.
At most, these can give you 12 VP. This isn't enough to wildly change how you approach a board, which makes them have lower impact than other Landmarks. However, like other Landmarks, it's important to at least stay even in the subgame they create.
You can divide the conditions into ones that are easy to satisfy, and ones that are hard. What exactly does that mean? I think of a condition as easy if you're guaranteed to move all 12 VP off the Landmark by the end of the game, and hard if that isn't going to happen. It's going to depend on the game, but Colonnade, Labyrinth, and Battlefield are often easy, and Baths is usually hard.
If the condition is easy:
It's pretty important to get 6 VP from the Landmark. If you only get 4 VP, and they get 8 VP, it's a 4 VP difference, and you need a Province to make up the difference. That can be a tall order on some boards, especially if the Landmark VP was easy to acquire.
To get to 6 VP you need to meet the condition 3 times. This doesn't mean you always go for the condition at every opportunity - it means you play in a way that makes it easy to meet the condition early in the game. The way this pans out in practice:
Arena: Overbuy terminals (but not too much), think a bit before playing your last Action.
Basilica: Get +Buy early and use it. (Note it triggers on every buy, so if you buy two $3s, you'll get 2 VP on your 1st buy.) If you can't do this, buying a $5 cost when you have $7 is the best case.
Battlefield: It's hard to balance this one, but roughly, a money-ish deck should buy Duchies over Gold after the 1st Gold. In engine vs BM, engine can afford to contest about 4 VP and try to make up the rest of the diff through a better deck. In engine vs engine it's very hard to judge.
Colonnade: Identify 2 key actions and focus on those in the early game. If you can get away with focusing on just 1 Action, even better, but you normally can't.
Labyrinth: Get +Buy or a gainer early and use them.
Baths is almost never easy to satisfy, so it doesn't get a section. Sometimes you have cards like Tactician to make it easier.
If the condition is hard:
Ignore it for most of the game, and treat it as an alternate source of VP in endgame. For example, gain Baths VP instead of buying Estate in late game. Same for Basilica.