Red Fortress: History and Illusion in the Kremlin. By Catherine Merridale. Metropolitan Books; 506 pages; $35. Allen Lane; £30. THE walls of the Kremlin keep foreigners out and secrets in. But Catherine Merridale has managed to get into this famous citadel and unpick at least some of its mysteries. Nobody reading her vivid and meticulous book, "Red Fortress," is likely to come away with much affection for Russia's rulers. Nor are they going to wish that they were transported back in time to the often squalid, mad and brutal eras she depicts. But they will begin to see why Russian history exerts such fascination on those who catch even a whiff of it. "Red Fortress" would be remarkable as an architectural history alone. Ms Merridale shows how the famous onion domes and towers took shape: who built them, how and why. She also relates how many buildings have perished: sometimes from fire and the sword, at other times because of rulers" vanity and occasionally because they were so badly built in the first place. The cycle seems relentless: treasure is extorted from the boyars and nobles (who in turn squeeze it from their groaning serfs). It piles up in the storerooms and treasure chambers - only to be stolen by its custodians, looted by foreigners or destroyed by moth, damp, rust and carelessness. Ms Merridale is a historian by training, but she has a detective's nose and a novelist's way with words. Her eyes and ears are as sharp as her pen. Even the secretive and taciturn denizens of the Kremlin that she speaks to seem to find her expertise and curiosity irresistible. She spots mouldering clues to the past as she trudges through musty corridors and cluttered storerooms. She skewers the way in which Russia's rulers twist and even invent history to glorify their rule. And she highlights the paradoxes and myths which beset the buildings round Red Square to this day. Vladimir Lenin's embalmed corpse, for example, lies venerated in his mausoleum just outside the Kremlin. Yet the church that he did so much to destroy is now revered as a pillar of the state. Ms Merridale's "Red Fortress" is a splendidly rich portrait of an exotic and puzzling redoubt.