(also big chanse you know these).
Round squares
This is a little example to make ''circles'' without world edit(cobble is nothing).
Sneak space
Place a door in a corner of a room and plase somethin unimportand behid it. It`s the perfect cove for a treasure room.
Magical stair
This is ideal for a secret room but very well known. Place a stair with a right angle on a wall and place on next to it to create a corner. The corner you made doesn`t exist in the game and so you can plase and use blocks trough it. If you aim a bit the outer lines will disapear. I connected it to a jeb-door but it is usable with any kind of redstone..
Nice fort
Give towers in a castle diffrend sizes for a better result.
T-flip flop
With this instalation You can send a redstone signal of exactly on tick (ore more if you want). You can connect this to a fast clock. Or you can connect it to a sticky piston. If you use it the piston will grab and release the block. Very handy for an on/off machine with a button.
Simpel but effective building tips
Has to be in the system