▁the ▁River ▁Med way ▁and ▁Wat ling ▁Street , ▁a ▁Roman ▁road . ▁It ▁has ▁long ▁been ▁assumed ▁that ▁the ▁first ▁castle ▁was ▁located ▁next ▁to ▁the ▁river , ▁just ▁outside ▁the ▁south - west ▁corner ▁of ▁the ▁town ▁walls . ▁The ▁conject ural ▁site ▁of ▁the ▁early ▁castle ▁later ▁became ▁known ▁as ▁" B ole y ▁Hill ". ▁Arch ae ologist ▁Tom ▁Mc Ne ill ▁has ▁suggested ▁that ▁these ▁earliest ▁cast les ▁in ▁England ▁may ▁have ▁been ▁purely ▁military ▁in ▁character , ▁built ▁to ▁contain ▁a ▁large ▁number ▁of ▁troops ▁in ▁host ile ▁territory . ▁ ▁According ▁to ▁the ▁Dom es day ▁Book ▁of ▁ 1 0 8 6 , ▁the ▁Bishop ▁of ▁Ro chester ▁was ▁given ▁land ▁val ued ▁at ▁ 1 7 s ▁ 4 d ▁in ▁A yles ford , ▁Kent , ▁in ▁compens ation ▁for ▁land ▁that ▁became ▁the ▁site ▁of ▁Ro chester ▁Castle . ▁Of ▁the ▁ 4 8   cast les ▁mentioned ▁in ▁the ▁survey , ▁Ro chester ▁is ▁the ▁only ▁one ▁for ▁which ▁property - own ers ▁were ▁re imb urs ed ▁when ▁their ▁land ▁was ▁taken ▁to ▁build ▁the ▁castle . ▁From ▁the ▁ 1 1 th   century ▁the ▁castle - guard ▁was ▁a ▁fe ud al ▁oblig ation ▁in ▁England . ▁This ▁often ▁took ▁the ▁form ▁of ▁kn ights ▁g arrison ing ▁cast les ▁for ▁their ▁l ords ▁for ▁a ▁set ▁period . ▁There ▁is ▁no ▁compreh ensive ▁list ▁of ▁which ▁cast les ▁were ▁ow ed ▁service ▁in ▁this ▁form , ▁but