。
The palace was under heavy guard.
Literal
Palace [topic-は] forbidding [attributive-の] vigilance-manner was-[polite-past-でした].
物々しい ('forbidding, ominous, imposing, heavy,' often with a military or ceremonial connotation) is a reduplicated adjective from 物 ('thing'). 警戒ぶり ('the state/manner of vigilance') uses ~ぶり ('manner, way of X-ing') as a suffix forming abstract nouns for the quality of an action — 話しぶり ('way of speaking'), 働きぶり ('work ethic').