The painting is rather naive, and purely artistically not worth the estimate by far, but it gets interesting nevertheless because the city in the background seems to be a rather correct depiction of Antwerp ca. 1600. This would make it a depiction of the 1584-1585 siege of the city by the Spanish troops. Whether the painting really is Dutch 18th century is hard to tell, 17th century seems more likely.
Pieter Meulener, Siege of AntwerpThe style of the work reminds somewhat of the works of Pieter Meulener and the like, but isn't good enough to really be attributed to any known painter. For a work depicting a generic battle, the price is too high: for an old (though probably not contemporary) depiction of the Battle of Antwerp though, it may well be justified; views of Antwerp sometimes fetch prices much higher than the artistic merit would justify.
RKD which I missed, but which confirms the " reminds somewhat of the works of Pieter Meulener" statement I made above: the work for sale is a copy of or variation of that painting, which was sold at Piasa (France) in 1997 for 130,000 FRF (near to 20,000 Euro). It depicts the Entrance of the Cardinal-Infant Ferdinand in Antwerp, which means that the people shooting at the "retreating troops" which baffled me, are probably soldiers firing blanks as a honorary salute. The Entrance happened in 1635, so the painting in any case is later than that! So, right city, right painter, wrong occasion; I can live with that ;-)
UPDATE 2: sold for 4,600 Euro, clearly below estimate