, ▁the ▁key ▁for tr ess ▁in ▁the ▁And alus i ▁line ▁of ▁defence ▁( the ▁T agr ) ▁against ▁Ur g ell , ▁which ▁he ▁successfully ▁took ▁by ▁storm ▁twice . ▁ ▁Er m eng ol ▁prompt ly ▁en fe off ed ▁Arn au ▁as ▁lord ▁of ▁À ger ▁and ▁charged ▁him ▁with ▁the ▁rep op ulation ▁of ▁the ▁dev ast ated ▁territory ▁between ▁the ▁Christian ▁and ▁Muslim ▁front iers . ▁To ▁this ▁end ▁Arn au ▁issued ▁char ters ▁specifying ▁the ▁terms ▁of ▁colon isation ; ▁Mont sec ▁became ▁the ▁centre ▁of ▁a ▁new ▁Christian ▁princip ality ▁in ▁the ▁south ▁of ▁Ur g ell . ▁For ▁his ▁primitive ▁ch anc ery ▁Arn au ▁employed ▁an ▁efficient ▁scri be ▁named ▁Vid al , ▁who ▁brought ▁the ▁convenient ia ▁( con vention ) ▁into ▁common ▁practice ▁in ▁western ▁Catal onia , ▁especially ▁through ▁the ▁work ▁he ▁did ▁drawing ▁up ▁convent ions ▁on ▁beh alf ▁of ▁Raymond ▁IV ▁of ▁P all ars ▁J uss à , ▁the ▁husband ▁of ▁Arn au ' s ▁daughter ▁Val ença . ▁Arn au ▁had ▁extended ▁his ▁influence ▁into ▁P all ars ▁J uss à ▁through ▁this ▁mat rim onial ▁al liance ▁( 1 0 5 5 ), ▁at ▁which ▁time ▁Raymond ▁had ▁p led ged ▁him ▁four ▁cast les ▁as ▁ins urance ▁against ▁his ▁breaking ▁his ▁marriage ▁v ows . ▁ ▁Arn au ▁controlled ▁over ▁thirty ▁cast les ▁and ▁numerous ▁other ▁agricult ural ▁settlement s ▁at ▁the ▁height ▁of ▁his ▁power , ▁just ifying ▁his ▁assumption ▁of ▁the ▁titles