Would launching space ships at the top of high places (like buildings or mountains) would have significant effect? What would be the height where launching would be less energy-/fuel-consuming than launching from the sea-level?
Is it even possible to create a building that is tall enough and even able to keep the weight of the space ship, the necessary rocket units (full of fuel) and the other necessary stuff?
Would launching space ships at the top of high building would have significant effect?
Not any standing building or tower. The highest of these come close to 1 km, so I'll use that. There is a certain "delta V" needed to increase altitude by rockets that amount. There are other factors, notably drag and gravity drag which I'll ignore for now. The delta v from the elevation change itself is:
$$ \sqrt{ g h } = \sqrt{ 9.8 \frac{m}{s^2} 1,000 m } \approx 100 \frac{m}{s} = 0.1 \frac{km}{s} $$
The often quoted value for the delta v to get to space is around $10 \frac{km}{s}$. So even with the tallest building, it would only save you on the order of 1% of your budget. This is underselling the factor somewhat.
The glaring flaw is that you could just go to higher altitude. There are nice flat planes in plenty of areas of the world over 3 km high. A tall building is obviously not a good alternative.
Some people do talk about a space pier, for instance. But these are built on technology that we've never come close to implementing and would require heights many times beyond what has been accomplished in building technology to date. I've read some papers that talk about the engineering required for the supports. They would necessarily entail active control. Super-tall masts have a history of breaking due to a combination of wind forces and structural resonances. In order to keep going higher, you first have to understand these very well, then you need to go into novel technology that will dampen modes of flexing and basically keep those members straight over long distances. These could be gyroscopic, but there are several approaches that could be used (even airfoils).
These super-tall tower approaches (if ever reasonable) would have to compete with other approaches of high-altitude balloons, or possibly even rotating space tethers (a space elevator alternative). I'm still asking around about high-altitude ionic levitation, but only because I can't yet figure out why it won't work.
Also consider the scaling factors involved. High launch points are much more advantageous for smaller rockets. With air drag, there is a scaling benefit. The larger your rocket, the less air drag subtracts from your delta v budget. A higher altitude of launch will make smaller rockets viable, which would not have been viable at sea level. That's nice, considering that your structural limits will restrict the launch mass anyway.
