I can assure you that the Castle is the strongest, but the Tower is not far behind. Regarding the economic efficiency of the Rampart, I can argue that the Stronghold (Barbarian) is the most economical; the Rampart needs a lot of crystals. I tested this extensively in the past under equal conditions. The Castle has the strongest units at levels 1 and 7 in the game, although the Halberdiers might be able to compete with the Centaurs. The cost is high, but it's balanced. The magic is above average. The other units are also quite good. The Tower has a very large magical potential, which is a big plus. Three archers are a plus. The units are not weak, but they don't have the strongest units either. The Palace for the Titan is simply a waste; however, the Giant is cheap!