Equal parts wonderful and awful.
I adore it for it is very tactically inclined indeed, and a very good strategy game through and through; however, the harder difficulties you play on, the more and more a single battle (that you cannot restart) can hinge on a single movement, and time after time I lost the opportunity to wipe out an enemy force because instead of attacking, my soldiers just meandered on closer to them, for I happened to click the square next to them by accident, and couldn't undo it.
I find this incredibly odd; for you had the foresight to realize that losing in the major battles meant you lost entirely; so you allowed for players to leave those and restart as if they didn't happen; yet for battles wherein you could gain gold and experience, you did no such thing.
So your options, as a player, are: "Win every battle, and gain more experience and upgrades thereby" or "Lose, and with every battle you lose, the game becomes more and more frustrating".
The 'oops' is another awful thing. Don't get me wrong, it's a fantastic idea; because blunders naturally happen in real battles. However, there ought to be a likelihood of that sort of thing. For example, you have a greater chance of missing a turn if your men are walking on ground that doesn't suit them (I.E, heavy swordsmen, in full armour, would have a harder time walking through a jungle than, say, the light infantry). And again, on the harder difficulty you play, the more awful it becomes; since the less and less troops you have, the MORE and more vital each of their attacks and movements becomes; so when you have your men missing out on attack after attack (usually after bringing themselves right next to the enemy and within range of their arrows), you're more likely to lose; more likely to lose means less money and exp; less money and exp means worse troops; and worse troops means more likely to lose. It is a vicious circle that continues until you simply CANNOT win, for the enemy's forces are so great and elite that you get completely wiped away with them.
My verdict: High difficulties don't make the game more challenging, just more frustrating. Do yourself a favour and play on Normal.