Spies do work, if you can mass them in sufficient quantities. I think fivefold angels were spies in addition to just being stealthy? Anyway, your huge research graph showed that you were aiming for something, and staying very quiet about it, didn't know what it was though..
But still, if you had waited for ulm to bump into your castle, you would've had much better chances since the gate is a very easily defendable choke point (as we saw with the fight in your cap
).