13th century Henry's successor, Richard I, paid relatively little attention to Kenilworth, but under King John significant building resumed at the castle. When John was excommunicated in 1208, he embarked on a programme of rebuilding and enhancing several major royal castles. These included Corfe, Odiham, Dover, Scarborough as well as Kenilworth. John spent £1,115 on Kenilworth Castle between 1210 and 1216,Allen Brown 1955, p.394, referenced in Cathcart King 1988, p.71. building the outer bailey wall in stone and improving the other defences, including creating Mortimer's and Lunn's Towers.Morris 2010, p.29, 37; Pettifer, p.257.Comparison of medieval financial figures with modern equivalents is challenging, especially with the larger sums of money used for projects such as castles. £1,100 could equate to from between £578,000 to £13m in 2009 terms, depending on the measure used. He also significantly improved the castle's water defences by damming the Finham and Inchford Brooks, creating the Great Mere.Hull 2006, p.132. The result was to turn Kenilworth into one of the largest English castles of the time, with one of the largest artificial lake defences in England.Pettifer, p.257; Thompson 1965, p.156. Because John had poured so many resources into the building of the castle and considered it an important strategic castle, he appointed household knights such as Robert of Ropsley to act as castellans. John was forced to cede the castle to the baronial opposition as part of the guarantee of the Magna Carta, before it reverted to royal control early in the reign of his son, Henry III. thumb|left|upright=1.4|Kenilworth Castle seen from the west; by the 13th century, the foreground would have been occupied by the water defences of the Great Mere Henry III granted Kenilworth in 1244 to Simon de Montfort, Earl of Leicester, who later became a leader in the Second Barons' War (1263–67) against the king, using Kenilworth as the centre of his operations.Platt, p.85. Initially the conflict went badly for King Henry, and after the Battle of Lewes in 1264 he was forced to sign the Mise of Lewes, under which his son, Prince Edward, was given over to the rebels as a hostage.Prestwich, pp.46–7. Edward was taken back to Kenilworth, where chroniclers considered he was held in unduly harsh conditions.Prestwich, p.47. Released in early 1265, Edward then defeated Montfort at the Battle of Evesham; the surviving rebels under the leadership of Henry de Hastings, Montfort's constable at Kenilworth, regrouped at the castle the following spring.Carpenter 2004, p.381. Edward's forces proceeded to lay siege to the rebels.Prestwich, p.56.
13th ceNtUry henry's suCceSsor, RiCHard I, Paid RelAtiVely LitTle aTTenTIon to KEnilWOrTH, but under kIng john sIgnificant buIldiNg resumed At tHe cASTLe. wHeN JOHn was ExcoMmuniCATed IN 1208, he eMbarked ON a prograMMe of rEbuILdIng anD eNHanCing sEVeral mAJOR roYAL castleS. ThEse InClUded corfe, ODIham, dover, scARboroUGh as WelL as kenILworth. johN spent £1,115 ON kenIlWorth CastlE betweEN 1210 anD 1216,allEn BRown 1955, p.394, reFerenced iN CaThcart kiNG 1988, p.71. buIlding the outeR baileY waLl In sToNe and ImPrOViNg The oTHer defEnceS, incLuDiNG creAting mortiMEr's AnD lunn's toWERS.morrIs 2010, p.29, 37; pETTiFEr, p.257.coMPaRiSOn Of Medieval FiNAnciAL figures with MOdErn EquIvalEntS is chaLleNGing, esPecIAlLy witH The lARger suMs of MOney UsEd FOR PROJEcTS sucH aS caStLeS. £1,100 COUlD eqUatE To frOM bEtwEen £578,000 To £13m iN 2009 tErms, depEnDIng on THe MeasURe usED. HE Also SignIFicAntly imprOved tHE casTle's waTEr dEfENceS By daMming tHE FInHam and InCHFoRd brookS, CrEatINg the grEat meRE.huLl 2006, p.132. The resuLt was to tUrn kenIlWortH iNTO one OF tHe laRgest eNGlIsh CAStles of The tiMe, wIth oNe of THe laRgEsT aRTiFiCiAl lAke DEfencEs iN ENgLaND.pettIFEr, P.257; tHompsOn 1965, p.156. becausE johN had PouRed so MAnY rEsourCeS intO The bUIlDInG oF tHe cAstLe AnD conSiDeRED It An important stRAtegic caStle, he ApPointEd housEholD KnIgHTs such AS RobeRt of ropsley To act as CasTeLlAns. johN Was forceD tO CeDe the CAStle to THe BaRoniAl oPposItion as part of thE Guarantee oF tHe MaGNa cArTA, bEFore it revErteD To ROyaL cOnTrOl earlY iN the Reign Of his soN, hEnry iii. thUmb|left|uprIght=1.4|kEniLWorth cAStle seen FRom tHe West; By thE 13Th cenTurY, The foregRound would haVe BeeN oCcupIEd By the WaTer defEncES of tHe Great merE HeNry iii gRANTed KeNILworth in 1244 tO siMON DE montForT, Earl Of leIceStER, wHO lAter BEcaMe A LeaDer iN tHe seCOnd barons' War (1263–67) AgAiNst tHe KinG, using kENIlWoRth As THe centre oF hIs OPeraTionS.plAtt, P.85. inITialLy THe cOnFlict wENt bAdly for kiNg HEnry, anD After ThE bAttle oF LewEs in 1264 he waS Forced tO siGn tHe mise of Lewes, unDeR which hiS soN, Prince EdWard, wAs GiVen ovEr to THe rEBels aS a HOstaGe.pREstwich, Pp.46–7. edwaRD was takeN back To KenilworTh, wHerE cHrONicleRS cOnsidereD hE was hEld In unDuly HArsH cONDitIoNS.PREsTwICH, P.47. rEleasEd iN early 1265, edward tHeN DefeateD MontfoRt At the battLe of evEShaM; the suRvivINg RebeLs UndEr THE lEaDeRShiP oF henrY De hAstings, MontFort's CoNsTABlE at keniLwoRth, regRouPed At THE cAstle tHe FoLLOwIng Spring.cARpENTEr 2004, p.381. edwARd's forceS ProcEeded to LAy siege to ThE rebeLs.prestwiCH, p.56.