There's some helpful tips on several of your questions in the forum FAQ:

https://forums.totalwar.org/vb/showthread.php?t=72109

In terms of winning battles, you might look at frogbeastegg's guide to RTW. Much of the tactical advice carries over.

Personally, I would advise specialising in upgrading one or two castles, so you can get the best troops soonest. Form a full stack army that is balanced - some archers, swords, spears and cav - and then follow "rock-paper-scissors" in battle. I tend to rely on softening up the AI with archers behind a shieldwall, then move in for the kill with swords and cav on the flanks.