Nice tutorials! I will recomend these to Blender n00bs the next time ;)

Oh you can do circle select... did not know this so far and again learned something new :-/

Something like this: 

,but with interiors pls, this sould keep you busy for a while.

Wow, making it low poly enough to make Duion happy would take a genius ;) On the other hand, it would be an interesting high poly model.

There is something like an 64 000 poly limit for one object in some engines, but you can seperate it.

Probably for such a model you would have a budged of 1 million polys or so.

You really shouldn't make such a building out of a single mesh anyways... there are so many repeating parts that can be represented by a single duplicated mesh in the GPU. The the rest you just block out with level geometry (given you have a proper level editor :p ).

If you will see something like that in a modern game it is probably build out of one mesh, at least each part of the house, you could split the towers and walls for example, the interiors will be the only real complicated thing.

Try to build it on the grid with standardized measurements like 4x4 m or so, this way it will be compatible with most engines/systems, like your last modules.

If you will see something like that in a modern game it is probably build out of one mesh, at least each part of the house [...].

I disagree. This might be the case for certain genres where reusing assets is not desired/necessary, such as arena shooters, but in modern open world games, structures and interior are most certainly built from flexible, modular pieces. See for example this excellent article that covers the level creation workflow at Bethesda.

This is for dungeons that are seperated from the outer world, so you have a bigger budget of items and polygons, but in an open world you want to keep your item count as low as possible and so try as good as you can to merge as much into one object as possible.

I still think it makes more sence to split such a structure into several modules, e.g. single towers/wall pieces, and simply merge all static meshes into one big static mesh when loading the game.

In the modular design with skyrim and similar games they split it up, you have an outside world and if you enter a dungeon, the dungeon gets loaded with its modules, but in the outside world, you don't have these modules rendered in most cases.

So in this case you would have a big mesh or some meshes of the castle outside and if you enter it the interiors get loaded.

Where did I say that you should render the interior if you're not inside? Actually, one of the advantages of a modular system is that you don't have to render one or the other if it isn't visible, andd that your interior can be larger than the exterior, which both isn't possible if you have one big manifold mesh as you suggest. Sure, a manifold mesh might save you a few hundred or even a few thousand polys over a non-manifold mesh, but that's literally nothing on modern hardware. Additionaly, only a small fraction of games is GPU-bound anyway; as far as I know, most games are CPU-bound. So why waste your artist's valuable time on something like this? And even if you later realize that you need to save those few polys, you can still take your modules and manually merge them into a single, manifold mesh; the reverse would probably be much harder.

In most modern games everything gets rendered all the time, there is just a switch between inside and outside.

But for flowing multiplayer gameplay you cannot make cuts and loading sequences and this is what I would want.