Can you tell me if there are any restrictions on building within the Castle?

Yes - all restrictions are only related to resources - you can build whatever you can afford... The conditions clearly state WHAT is available at the start. And nothing else will appear in the next two weeks...

Added 7 minutes later

2. Nothing is said about the income in the Castle and what buildings can be built at all.

I thought that since it says "there is one Castle," its contents could be up to the player's discretion - maybe that's the "catch"?

Also, it doesn't say what artifacts the player has - we decide that ourselves...

And regarding one player, that's probably not correct. If we have money, we can hire as many players as we want. The rules of the game are not limited, only the starting resources are. This is the essence of the task. In general, the author should have formulated it more clearly so that there would be no discussion in the thread...

Otherwise, the task will be reduced to something primitive.