The deficiency of the written sources obliges to refer nearly solely to the topography and to the archaeological vestiges to reconstitute from this moment the evolution of the castelnau.
Because, he agrees to underline it, the foundation of Pierre of Lobaner was a success since, to the earliest in the middle of the XIVème century, at the latest at the end of the following century, Mont de Marsan had become a double city, that one even that reveals us the plan of the beginning of the XIXème century: the castelnau and his excrescences cave Twelve and Midou - 500m on 200m, either 10ha about - and the Borough to the south of the Midou 250 on 160 m in relation to the Midou and 225 to 375 m in border of the Midouze, either more or less 6 ha.
If he is certain that such were the limits of the agglomeration at the end of the Middle Ages - what one can deduct the tracing and the elevation of the defensive wall as they appear on the drawing of J. of Weert - on the other hand, pose the problem of the stages of his development.
