[Norfleet]

As the canonical castle spammer, I favor the watchtower, due to its low cost in gold, time, and points. Admin rating is somewhat secondary when your intent is to plug every single province on the map with them, rendering annoyances like Ghost Riders ineffective against you.

Furthermore, the defensive layout of the Watchtower is top notch: No bothersome walls to impede the exit of your forces, since they're obviously too stupid to stand in the gate and not move.