Under its eponymous founder, Osman I, the Beylik of Osmanoğlu expanded at Byzantine expense south and west of the Sea of Marmara in the first decades of the 14th century. With their annexation of the neighboring Beylik of Karasi and their advance into Roumelia as of 1354, they soon became strong enough to emerge as the main rivals of Karamanids, who at that time were thought to be the strongest. Towards the end of the 14th century, the Ottomans advanced further into Anatolia by acquiring towns, either by buying them off or through marriage alliances. Meanwhile, the Karamanids assaulted the Ottomans many times with the help of other beyliks, Mamluks, Aq Qoyunlu ("White Sheep Turkomans"), Byzantines, Pontics and Hungarians, failing and losing power every time. By the close of the century, the early Ottoman leaders had conquered large parts of land from Karamanids and other less prominent beyliks. These had a short respite when their territories were restored to them after the Ottoman defeat suffered against Tamerlane in 1402 in the Battle of Ankara.