Maximilian immediately attacked the conquested territories in Austria in 1490. The Black Army fortified itself in the occupied forts on the western border. Most of them were captured by trick, bribery, or citizen revolt in a few weeks without any major battles. The trenchline along the river Enns, which was built by mercenary captain Wilhelm Tettauer, resisted quite successfully for a month. Due to the lack of payment, some of the Black Army mercenaries, mostly Czechs, switched sides and joined the Holy Roman army of 20,000 men in invading Hungary. They advanced in the heart of Hungary and managed to capture the city of Székesfehérvár, which he sacked, as well as the tomb of King Matthias, which was kept there. His Landsknechts were still unsatisfied with the plunder and refused to go for taking Buda. He returned to the Empire in late December but left garrisons of a few hundred soldiers in those Hungarian cities and castles he occupied.