First time making a castle cake and i need some guidance please.

Have to put two turrets on top of the cake and then one on each side of the cake worried the two at the top in particular will fall down when delivering.

Was going to going to use dowelling in the middle of the top turrets and simply stick the side ones to the cake with royal icing. Any thoughts on how best to do it?

Royal icing or white chocolate with your dowles for sure. I experienced this first hand thinking buttercream and dowels would be enough. It was not. I have a photo poated and you can certainly see the separation. Once they are attached you can go back and cover with buttercream to cover the seams. Good luck!