Indeed, one assigns to Gaston Phébus (1343-1391) the construction of the castle of Nolibos (in Gascon, "No-li-bos" means "you the doesn't want" there, implied the enemy) that occupied until the beginning of the XVII ème century the angle northeast of the surrounding wall.
However, this castle is baffled only of about twenty eastward meters in relation to the north-south alignment of the dungeon of the Cataye.
He is not of the quite impossible, besides, that the castle of Nolibos has been raised in the ditches of the surrounding wall of the XIIIème century. Perfectly recognizable on the drawing of J. of Weert, he understood: a tower squared close to the door of Roquefort, a dungeon constituted of coupled 3 towers, of big buildings dominating the ditch, finally, to the angle northeast of the surrounding wall, 2 other towers.
The construction of the castle of Noliboses and courtines that prolonged it southbound until the Midou had a determining influence on the ulterior development of the city in this sector.
Considering their importance, the fortifications conferred a definitive character practically to the limits from the agglomeration to this place. One could wonder in these conditions that one didn't benefit from this to include in the new walls sit it of the Clarisseses transferred from Beyries, after 1275, to 150m the castle of Nolibos, on the site of a former Saint-Jacques hospital.
But one must realize that to height of the castle of Nolibos, the Twelve takes a direction northeast and the Midou a southeasterly direction; the widening of the interfluve would have permitted a comfortable development of the city in this direction, but the plotting, more to the East, of the new defensive wall must appear useless to the constructors of the XIVème century, considering the importance that to this time had taken the borough situated on the other strand of the Midou.
Such appear us to have been the main stages of the development of the castelnau of Mont de Marsan. So, to evoke them, we inspired ourselves data of the topography and those of archeology solely, it is that we don't arrange for all the medieval time that of only one document susceptible to illuminate us on the urban landscape of the city: the role of the "sirmanatge" or fundamental tax of 1450 and 1452.
However, its interpretation proves to be of the most delicate. One raises, indeed, indeed, between Twelve and Midous: a" Borough Nau", with 2 "carrères (63 recognitions)"; a" Borough Bieilh" (54 rec. ); a district" daffore carries" it (7 rec.); while coming back inside, a ("pass carried it deffentz") 1st district where one raises 24 recognitions then, while turning" near the ostau of Arros" another district (28 rec.) ; while passing between the walls of the city another group of 6 recognitions; finally, in the Cataye of Pujaurin (The Cataye of Pujolin) a last district where one raises 24 recognitions.
The existence of an Old Borough and the one of a New Borough translates, undoubtedly, the opposition between the primitive castelnau and one of its excrescences: but the existence of an Old Borough inside the defensive wall of 2 other districts, the one of the Cataye and probably the one of Nolibos, reveal a more complex evolution than the one that is suggested by the exam of the plan. He/it doesn't make any doubt that it existed there, otherwise, a small suburb probably to the door of Saint-Jean-d'Août. With its 212 recognitions for more of 10 ha, the castelnau presented a fundamental structure different of the one of the Borough where, for 6 ha, one raises 283 recognitions.
This opposition that meets in the amount of the royalties - 4,51 d. in the castelnau on average and only 1,69 d. in the borough - appears again in the fragmentation of the beginning of the XIXème century. In the middle of the XVème century, the castelnau was, without dispute, the richest district of Mont de Marsan.
