Hi.
1: Subjects are basically what it costs to upgrade buildings and recruit units, along with the three main resources. You need to upgrade your farm to gain more subjects (the farm feeds the subjects).
2: You can conquer more castles by sending silver with your attacking units to a castle. You need a certain amount of silver to capture a castle. In order to get your first castle you need 1000 silver. You get silver by trading at your keep. Silver will cost a certain amount of resources, depending on the level of your keep. I suggest upgrading the keep to level 10 to get the best out of the silver trade.
3: You can get gold in various ways. In the tavern there's this option called "watch videos". I believe you get something like 2 gold coins per video you watch. Alternatively you can buy gold with real money. They do have special offers at times. Just remember that you don't really need gold to play the game, it merely makes it easier to play.
HTH

Hi.
Yes, subjects only get more as you upgrade the farm. Just note that they will decrease as you recruit units and upgrade other buildings, so keep upgrading your farm wherever possible. As for units vanishing, my only guess is that you're probably farming on free castles and sometimes their defenses get one or two of your units. Unless you're getting attacked...
@claudiogaranzini, I know someone who tried playing it on android and gave up for the same reasons; it seems as if that part of the game is inaccessible on android. Perhaps you guys can try and contact the devs about this? From what I understand they cooperated in enhancing the accessibility for the game on iOS, so perhaps they can fix it for android also.

thank you all! Is there a similar game that has a massive multiplayer like this and that's accessible also for android users? Can any of you suggest some title? Thanks again!!

Yes, you need to do some research in the library. I think it's crop rotation and seller storeroom, but correct me if I'm wrong. Fortunately on each research it will tell you exactly what it does.
Oh and don't forget, upgrading your library will unlock more research.

Hi.
Yes, all the buildings have limits. For example, the wood, stone and ore stores, as well as the fortifications, can only go up to level 20; the production facilities and arsenal can go up to level 30; the library, keep and tavern can go up to level 10 and the market to level 8.

To see more castles, swipe left, right, up or down with 3 fingers. When you're done, tap on the text that says "* units away from your castle". your castle is your castle's name.