Philip's involvement, however, it was stipulated that both he and Joan would renounce the crown as soon as their heir, male or female, reached the age of 21, or they were obliged to pay a fine of 100,000 livres tournois. Placing a limitation on the monarchs' reign was an unprecedented and extraordinary condition. Domestic policy Grandson of a king but raised with the expectation of being merely a count, Philip took kingship seriously. He was completely unprepared for the role but succeeded, all whilst counselling the newly enthroned King of France, in learning within a few months everything he need to know about his new kingdom, its people, language and institutions. Philip III proved to be an effective and successful ruler. His most famous achievement was the improvement of the kingdom's law code in 1330. In September 1331 he and Joan moved back to France; they returned to Navarre in April 1336 and stayed in the kingdom until October 1337. Altogether, Philip III and Joan II stayed in the kingdom longer than any Navarrese monarch since 1274. The couple spent most of their time in their various French fiefs and in their Parisian palace, from where they were able to further their positions as French princes of the blood, but Philip ensured their visibility in Navarre during several visits. He even visited the kingdom on his own on several occasions, while its "natural lady" administered his own counties in France. Philip III and Joan II's joint reign was marked by their close cooperation, but he prevailed in legislation and several other fields of government. Forty-one surviving royal decrees were issued in the names of both Philip and Joan, but Philip signed thirty-eight without referring to his wife, while only six were issued in Joan's name alone. The couple ensured that the Navarrese customs and laws were respected and that the kingdom's citizens were more directly involved in governance. Their active involvement in the administration of Navarre despite their absences, as well as putting the interests of Navarre ahead of those of their French homeland, earned them praise by both their contemporaries and by historians. Economic development projects undertaken by Philip and Joan include the construction of an irrigation system in the arid fields of Tudela in the south of their kingdom, reparation of castles and erection of a new one in Castelrenault. The king and queen were also determined to