Question: I'm making a game with target (orbit camera). I want to limit camera movements when in room. So that camera don't go through walls when moving around target and along the ground.
What are common approaches? Any good links with tutorials? I use Unity3D, nevertheless I'm good with a general solution to the problem

Answer: 
Make a snake game with it. Make a shooter game with it. Make some simple well known games to start off. Once you've done this, you should be more confident with your engine, and making these games will give you ideas on where to keep going.
Proceeding with levels seems like a good next step, it can teach you how to create data structures to fit your game, and file input/output.

Also, I would just have tried to build the engine with a set target game in mind. You don't have to be very specific, but say just make the engine for a specific genre that you like. When you know what to make with your engine, you have a better idea of what actually works in your engine and what doesn't. A good engine would have been "battle-tested" already, and usable in real-world cases. I have been making a game around sample framework code- as I added more features to it that the game would definitely use, it became clearer which parts can be re-used for future games.