In every Tower Defense game you build towers that developers created. In some games, you can choose tower foundation and gun seperatly but it's still not a real tower creation.

Do you think Tower Defense games should give players the posibility to create own towers and then use them on the battlefield? (Like creating own ships in "Leviathan: Warships")
How should it look like? Of which parts should the tower consist?

Its not a bad idea but the design of a tower isn't overly complex (as it is when creating a ship). Other then height, firepower and support or passive skills (as techno mentioned), its kind of like building a character that doesn't move.

That being said I have seen a few 2D app style games that unique branching design systems where players' choose branching expansions to house further weapons and systems to help fight off enemies from up down and either side. Another aspect to explore is the power required to fuel the tower's systems (resource collection and refinement) and maintaining the challenge of a "not in my backyard" mentality of the surrounding (possible growing) population that your tower protects with aesthetic and none practical upgrades.

What kind of tower creation do you want if selecting a base and gun aren't tower creation?

I mean that it isn't enough.

I think one could use a requirement system for the tower creation. Similar to Space Empires series ship creation, the ships have minimum needs that take space and needs to be fulfilled. A layered grid should be sufficient for a tower. (Imagine houses. Grids are rooms and layers are floors.)

Say a tower needs power supply/intake, a targeting system, rotation engine.

To create a tower, the player must add these required items to complete the creation process. They could easily also have different types. f.ex. one targeting system could be able to aquire targets faster, but would sacrifice range to accomplish it, while another targeting system could do it the other way around.

After the necessary requirements are fulfilled, the player could then proceed to add other items such as armor, weapons, passive stuff, shields.

A method like this is fairly simple and allows for extremely large number of different towers.

Defender's quest is an RPG with towers.

If players can build their own towers then you will face some serious balancing problems. Also it's quite stressful to design mid-combat. If I would make designable towers, I would let the players design them out-of-level (between levels). Then they could have a set of tower blueprints which would be implemented in-level.

a very important aspect in such games is game control being Deterministic that means player should know what exactly he should expect from what he builds , as in usual TD games you exactly knows what tower does what and in what range and in what shape! well if you design a system that let players to design their own towers its very unlikely to get such result if you do so you are a genius , but just ignore it if you don't have the idea. stick to the fact Determinstic , its like jumping in a platformer game , player doesn't expect character jumping to random height , he expect to jump as high as he wants ( based on time he keeps touching the screen for example).

but you could just limit the idea in very smaller range of possibilities maybe then you could have the enough control over the gameplay

In every Tower Defense game you build towers that developers created. In some games, you can choose tower foundation and gun seperatly but it's still not a real tower creation.

Do you think Tower Defense games should give players the posibility to create own towers and then use them on the battlefield? (Like creating own ships in "Leviathan: Warships")
How should it look like? Of which parts should the tower consist?

As few parts as you can. The number of possible combinations skyrockets the more possibilities you add...and that's a lot of content to develop and test.

Let's say a tower can have:

• 5 types of foundation
• 5 types of ammunition
• 5 types of armor
• 5 types of gun barrel

That means there are 54 different possible towers the player can make. That's 625 possible combinations. If you add another slot with 5 options, that becomes 55 or 3,125 combinations. As you can see, this scales up pretty fast.