Apple accused the European Commission of misunderstanding its business on day two of the iPhone maker's appeal against a $14 billion tax order.
The dispute, which is key to the EU's drive to collect more taxes, could run for years.
The case centres on tax rulings granted by Ireland to two Apple businesses in the country, Apple Sales International and Apple Operations Europe,
These rulings reduced Apple's tax burden for more than two decades - to as low as 0.005% in 2014.
The European Commission ordered the US company in 2016 to pay €13 billion of taxes it said were owed to Ireland.
But Apple and Ireland, whose economy benefits from hosting a number of multinational firms, are appealing against the decision at Europe's General Court, its second highest.
Central to the dispute is the importance of the Irish businesses, with Apple lawyer Daniel Beard arguing today they were not as significant as the Commission has asserted.
"Yes, Apple CEO Tim Cook said there were decisions taken in Ireland, but not strategic decisions," he said, referring to Cook's testimony at a US Senate hearing in 2013 which formed a key element of the Commission's case.
Commission lawyer Paul-John Loewenthal rejected criticism that the EU executive had not taken up Apple's offer to visit its operations in Cork, saying this was not necessary.
"What would a site visit accomplish?" he asked.
Ireland ended the tax rulings when Apple's two Irish units changed their structures in 2015.
The stakes are high for European Competition Commissioner Margrethe Vestager, who has spearheaded the European Union's drive against tax avoidance by multinationals, issued similar tax recovery orders to Amazon and Starbucks among others.
She also has other companies in her sight.
Vestager, who will stay on in her post for five more years but with new powers to set rules, will get a sense of how the General Court views her crackdown on September 24 when it rules on her tax recovery orders to Starbucks and Fiat Chrysler Automobiles.
The court will rule on the Apple case in the coming months, but the losing party is likely to appeal to the EU Court of Justice and a final judgment could take several years.