I've started playing the game and after the encounther with theDragon Knight, I went to the tower where I was supposed to go. After opening some gates , I entered one door. Inside is a ghost of the blacksmith and some stuff. The problem is, that I cannot go out, because the door cannot be opened form the inside, at least I haven't found any means to do it. Am I effectively trapped, or is there something to do?

Also, can i somehow access the chest in the farmer's (Johnson?) dungeon? That one behind the bent door, which doesn't react to the cursor, so it probably cannot be moved. I have found a key in that dungeon, but I couldn't find any use for it.

****spoilers****

Do a quick save. . .
( crank your health up before doing this if you can)

Walk up the fallen pillar and jump into the room of fire. Go quickly to the right rear corner, pull the chain, turn, pick up the sphere and jump out of the room .

That should open the door, if not pull the chains in the remaining rooms which are located near the green flames. You can jump directly into the room but I preferred jumping from the pillar as I landed closer to the chain pull.
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Farm celler room.
Standing at the door to the room, make your way around to the exterior right wall of the room by following the cellar halls. You will see shelving extending out from the backside of the wall. You will go thru a huge spider web as you start your approach to the wall. In the center of the wall ledge there is a pressure plate that must be pushed in order to reveal the room. See attachment.
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Edit: see attachment, "room of fire" may be a vague description . . =) Also I believe the green flame at the other chain pulls signify that the chains should be pulled which is a clue to look in the fire room for a similar chain. (not sure if they absolutely have to be pulled as I pulled them all prior to the one in the fire room)

Seems the game uses a system of trapping you as a way to insure you complete every task in the area. I have found several rooms that require tasks to be completed before exit is allowed. I'm only at Maxos's tower but the test rooms act the same way.

Also the colors of objects / platforms give clues as to how you should proceed. Lovis's tower was perplexing until I realized the blue tint of the platforms were showing a path.

There is also a chest in the room that is on fire. Jumping to it will keep you a little bit out of the fire, and if you are quick you can get the random crappy loot (possibly a healing potion to use after you pull the chain and jump back out of the room).

Thank you both. This has discouraged me from exploring, when it comes to main quests.

Just save periodically, especially before entering new building or caves, etc. Worse case scenario you can always reload loose a few minutes playtime.

If you tend to put off main plot quests until you have explored or done other side quests, you shouldn't get stuck somewhere at too low a low level without warning, so that should minimize any reloading you would need to do.

I didn't even realize I was locked into the room at the top of Lovis' tower, since I ran around looting and pulled all the levers to see what they would do. Also, I saved and used the scrying stone myself there, so it wasn't until someone asked in the Larian forum about how to get out of the room that I knew the door had been locked.

I didn't know that those were levers, so I haven't tried to pull them. :-[

I was looking at one of the murals, or something, when I noticed the chains were selectable. There were also similar chains in caves earlier in the game, though more obvious, being right beside doors, etc.

Same here, I've played through it twice and I never even knew you could get locked in that room.