From Chris Johnson...
Battle Tips
Completely surround and destroy vulnerable stacks. If you wish to force
a leader fate die roll on a monarch in battle by eliminating any possibility
of retreat, completely surround the unit(s) and attack. The main advantage
of this lies with magical leaders, such as The Eaters of Wisdom, who almost
always retreat (on a roll of 2-6). You have nothing to lose by doing this
to any loose enemy troops standing around as well. If you leave them alive,
they may stack with someone later and cause you headache.
Only make necessary attacks. Do not attack unless it is necessary; period. If the attack is needed for a situation or for your overall strategy, then do it. Otherwise, you are taking an unnecessary gamble.
Only attack when you know you will most likely win. This may not always be possible. If you are in dire straits and losing the game, sometimes a lucky battle can help turn the war. In general however, it is not wise to attack unless you think you will win.
Use numbers to your advantage. Keep in mind the battle system and at what number your stack will get a +1 to a die roll. For example, don't send 9 troops to attack a stack of 5 troops if you can afford an extra man. That one troop gives your die roll a +1. Better yet, send two troops in case you lose one in battle, so your +1 won't be lost.
The same technique applies to sieges. Also remember when surrounding a castle with, say, an intrinsic value of 4 that 8, 9, 10, or 11 troops give you the same bonus to plunder. If you have 12, by all means use them, but if not, only use 8 so that the other 3 can move normally. Of course, as stated above, having extras doesn't hurt if you are concerned about losing some of the forces to enemy attack.
Another thing about sieges is when completely surrounding a castle, use forces in a staggered order. That is, put country A's troops in the northern hex, country B's in the northeast, country A's or C's or mercenaries in the eastern, and so on. That way, if bad omens are rolled, you will still be able to make a plunder roll this turn. Also, barbarians are good for this as they aren't affected by bad omens.
Also remember that attacking a smaller force can sometimes cause you to lose more forces. Consider this example: your opponent has a stack of three troops, you have five. If you attack, it will be even odds (no pluses or minuses to either side). You roll a 6 and your opponant rolls a 6. You lose five troops while your opponant loses three. Had you only sent three into battle, you would only have lost three. The same applies when retreating. The rules allow for any part of a stack to retreat. Retreat only those that you don't want to lose under any circumstances.
Use terrain to your advantage. It is true that occupying mountain or
mountain pass hexes gives better defensive posture, but occupying non terrain
hexes can block your opponent and slow him down considerably. If you have
a battle going on and troops to spare elsewhere, use them to block hexes
to the battle so your opponent cannot easily provide reinforcements.
Defensive Tips
Avoid movement with monarchs unless you know it is safe. You must always
assume that any leader fate die roll for a monarch will fail. Never leave
your fate in the hands of luck.
Earthquake your own castles. If your opponents has your 40 point castle surrounded and near collapse, and you just happen to have a unit near Graystaff, earthquake the castle and rob your opponent of the points. There is nothing in the rules to prohibit this. It's dirty, but effective.
Activate sea borne countries if your opponent(s) have a lot of armies. If you are on the defensive, and your opponent has many armies, activating countries with ports can be safe because your opponent has to have fleets to plunder your castles; his armies are useless against them alone.
Take control of Graystaff. Raise some barbarians and surround the Altars, permitting entry for only your troops. Otherwise, prepare for firestorms.
Thin out your troops under the threat of firestorms. A firestorm can
only affect one hex, so by thinning out the troops, you reduce your chances
to lose them. Keep them within one hex of each other though, in case you
need to stack them quickly in defense. However, if there is no threat from
Graystaff, it is better to stack them as high as possible.
Game-Strategy Tips
Take control early. This is probably the number one thing to keep in
mind in this game. You win by controlling the game. The first 5 turns are
the most crucial. If you can get control immediately, you have a much better
chance of winning the war.
Think ahead. Remember, it is a 20 turn game. It is easy to only dwell on the immediate situation. If you have such an uneventful turn that you can do anything you wish for diplomacy, use it to prepare for the later parts of the game. For example, if you are on turn 5 and so far the game has been dominated by fleet warfare, take a diplomacy phase to activate a country full of armies, such as Zorn. You will most likely need them later and already having them will give you the advantage.
Make your opponent fight you on multiple fronts. If his countries are centrally located, do your best to control the countries all around him, to divide his forces. Or, if he has a port or two, activate a fleet power and attack the ports--right in the midst of heavy army warfare. This will weaken him by dividing his attention and resources.
Try to build up back-up forces. Never rely solely on reinforcements through the events rolls; you will lose as much as you gain doing this. Even during the heat of war in a certain area, activate countries farther away (if you have the chance) for later reinforcements.
Remember that you cannot score points on battles. It is easy to spend
the game in the glory of battle, gaining only a few points, only to lose
a monarch to a leader fate die roll later on and lose the game. Go for
the castles.