Object rearrangement.

What exactly is being cut down?

In multiplayer, you play with what you get. You figure out how to use a particular secondary skill or item, or not use it at all. And so on, you can go on and on.
As for restructuring, it's all very simple - each new castle means + to income and + to troop growth.

For now, I don't see the need to drag convoys with reinforcements to hold a castle.

First of all, it's not convoys, but chains of heroes. By the way, the ability to buy everything from one castle also simplifies the game in a silly way.