The most important thing is to study the lightning technology of building VT. When it is completed, the lightning soldiers and hidden knives can be built early.
It's the rightmost building on the second side, the second building from top to bottom.
Before, we had to build a barracks BG (a building in the middle of the first face), use the Dragon Riding Range BY to study the building, and then use the speed-up VC to study the building (the third building in the first row on the second face).