English Mastiffs were indeed used to guard country estates and so forth. Another common guard dog would have been the Bullmastiff.

English Mastiffs are really no longer suitable for guard duty as they've essentially had the aggression bred out of them over the years. They are WONDERFUL dogs - big, fuzzy foot stools, but with their size comes the predictable food intake, vet bills, and *flatulence.* An English Mastiff breaking wind is positively room-clearing!