Not a bad concept, but way way too easy, the tech trees are a little silly (I mean seriously, who didn't research Frigates and galleons). Being able to build a sloop in the same time as a Galleon is dumb, as is sailing with that unit the same turn I built it.
It seems as there is a great gulf between starting> Mid game> getting everything *except* 500 of each stand, it takes more effort to go from 400 of each to 500 than it took to go from 0-400 5-10 times. Give us something to do in the interim?
Sort os sad such a great game is laid low but such low level bugs regarding masters disappearing, it happened to me and it is very frequent. With the simple bugs fixed this game could be a 4.5+ and on the front page, no doubt. I would highly suggest you fix those bugs, it is keeping your good game from becoming a great game.
How to win:
Pick Zombie Knight, get the following attributes (ideally) 100% crit, any counter, +sp on crit, +%hp on crit, 100% stun, +100% poison.
The zombie knight has an triggered aoe effect that poisons, this effect will auto crit, stun melee attackers entering it, and proc the heal and +sp. Due to the nature of poison, anything entering will die in 3 seconds. You will never run out of mana and your hp will stay topped except on very high difficulties.
Your counter ability, stun, and poison will finish off any ranged attackers that hit you using a similar mechanic (poison)
Place enough stats in intelligence to cast the field 3 times, enough agility to get to 100% crit (with items) and the rest in vitality.
Congratulations, you can pretty easily beat any difficulty, use hearts if for some reason you get popped by a slew of ranged attackers or caught off guard.
The sooner you figure out that in the end game the "warrior" class is generally useless and irrelevant, the better. Chain lighting, mortars, and elves are pretty handily the best way to perfect the levels.