In the part of the movie when the Angels are fighting Eric Knox and Vivian Wood, they jump off the castle after it is blown up. Bosley then comes driving up in a blue jeep. But how did he get the jeep when he was brought to the castle by Vivian Wood and the Angels arrived in a boat?
They probably did have vehicles at the castle...just because you didn't see them until that point doesn't mean they weren't there. He probably found one and stole it..simple as that.