These designs in Castle construction were structures with two or more defensive curtain wall systems - with one higher than the other (generally the inner wall). The name "Concentric" is a misnomer. Designs of defenses of this type do not necessarily have to be circular in design. A rectangular castle could have a concentric wall system as part of it's defenses. Concentric Castles resemble "one castle inside of another" (by outward appearance). This creates an "inner and outer Bailey" (by default). Sometime, free standing Towers were used instead. This design was seen as far back as the Ancient Assyrians.