Without question, the best free-to-play browser game I've ever played. To give a rough outline of the game, you play as a family of characters in a fictional medieval world that's divided into several large islands. Every island is split into numerous regions that are ruled by different realms.
The realms then battle for control over the islands, although play generally reflects real world politics, so alliances will form and particular realms will have certain strategic aims they want to fulfil (e.g. taking a specific city or amount of land), and there will be gameplay cycles of war and peace.
The strategy gameplay is entirely team based and requires teamwork and tactical co-ordination between the players in a specific realm - lone wolves won't win battles (although there is the infiltrator character class who can work solo as an assassin and specialist in the 'dirty tricks' aspects of warfare). Each realm forms its players into armies, and some players take on the roles of generals or marshals who plan out the campaigns and normally issue orders. You can play as various different classes of character - warrior (with sub-classes of Hero and Cavalier), infiltrator, diplomat/ambassador, courtier, or priest. There's also an adventurer class that's almost like a unique game on its own, and plays differently to the 'main' game that's played as characters who are members of the nobility.
Battlemaster is turned-based, with two turns a day. So the playing speed is fairly low-stress and open to people who are busy in the real world. There's also a roleplaying element, but that's either as light or as heavy as you want to make it. It's mostly text-based and has a retro feel, although it does have some nice playing maps. Don't let the retro visuals put you off until you give the game a fair trial period to see if it grips you (say a couple of weeks playing time - remember that it's an intentionally slow-paced game).
For anyone who wants to play, I'd strongly recommend doing the Tutorial before plunging into the game itself. It's a fairly slow burning game that you get the hang of the more you play, and the vast majority of players will be more than happy to help new players. Don't be put off if you don't like the first realm you select to play in. There are a huge number of different realms, and all of them give different playing experiences.
I'm currently playing in Westmoor and Perleone on the East Continent; Kindara on the Far East Island; and Suville on Atamara; and I'd be happy to give pointers to any new players who appear. At the time of this posting, Kindara is in a pretty enjoyable war involving an alliance of so-called Free Realms on one side, with an Evil Empire on the other side.
Best of all, it's 100% free to play forever and there are no microtransactions or any other pay-to-win features.