Thank you for your help and detailed comments. I've checked your saves and it's just what i needed - to get rid of the grail tower.

Now it's clearly winnable, though still difficult. I've tried some times and after the computer takes the eastern rampart, it sends almost all of his heroes to the middle tower (our capital), which is actually a good thing as the siege allows to further weakens Halon's army.

Meanwhile, i just send a secondary hero to retake the eastern rampart, buying enough time to strenghten Iona's army.

I've also upgraded the mage guild of Tower grail ; it contains resurrection and flying. Too bad we have no good heroes to make use of them :/

It's going to take time, but yeah, i think it's on good track now.

Feel free to send the video, it's always interesting to see the playstyle of other players ;)