I haven't been able to confirm this by any other references but many of the older topiary forms are in fact distinctly soldier-like. The topiary at Levens Hall in Cumbria, shown here, is some of the oldest in the world, dating to the late seventeenth century, though it has been re-cut over the years.
Older English topiary work tends to be strongly anthropomorphic; Levens also featured forms representing Queen Elizabeth I and her ladies in waiting, attired in bulbous green hoop skirts. The twelve apostles in yew were a perennially popular theme. The Asian topiary tradition, on the other hand, is distinctly different than that of Europe--the favorite motif being cloud-like forms--and topiary has a much less significant place in youthful American garden history, which has no castle antecedents, than in the ancient traditions of England and France, which was the source of the sentinel reference.
Perhaps it is why topiary still seems so appropriate at gates and entrances, stiffly standing guard.