Given the following context:

By the summer of 1906, Mahler had been director of the Vienna Hofoper for nine years. Throughout this time his practice was to leave Vienna at the close of the Hofoper season for a summer retreat, where he could devote himself to composition. Since 1899 this had been at Maiernigg, near the resort town of Maria Wörth in Carinthia, southern Austria, where Mahler built a villa overlooking the Wörthersee. In these restful surroundings Mahler completed his Symphonies No. 4, No. 5, No. 6 and No. 7, his Rückert songs and his song cycle Kindertotenlieder ("Songs on the Death of Children").Until 1901, Mahler's compositions had been heavily influenced by the German folk-poem collection Des Knaben Wunderhorn ("The Youth's Magic Horn"), which he had first encountered around 1887. The music of Mahler's many Wunderhorn settings is reflected in his Symphonies No. 2, No. 3 and No. 4, which all employ vocal as well as instrumental forces. From about 1901, however, Mahler's music underwent a change in character as he moved into the middle period of his compositional life. Here, the more austere poems of Friedrich Rückert replace the Wunderhorn collection as the primary influence; the songs are less folk-related, and no longer infiltrate the symphonies as extensively as before. During this period Symphonies No. 5, No. 6 and No. 7 were written, all as purely instrumental works, portrayed by Mahler scholar Deryck Cooke as "more stern and forthright ..., more tautly symphonic, with a new granite-like hardness of orchestration".Mahler arrived at Maiernigg in June 1906 with the draft manuscript of his Seventh Symphony; he intended to spend time revising the orchestration until an idea for a new work should strike. The composer's wife Alma Mahler, in her memoirs, says that for a fortnight Mahler was "haunted by the spectre of failing inspiration"; Mahler's recollection, however, is that on the first day of the vacation he was seized by the creative spirit, and plunged immediately into composition of the work that would become his...

answer the following question:

Since 1899, what had been at Maiernigg? 

a summer retreat