Cryptocurrencies such as Bitcoin have been one of the new major technologies of the last decade. In this paper, we assess the security of Bitcoin using attack-defense trees, an established formalism to evaluate the security of systems. In this paper, our main contributions are as follows: (1) We provide an extended attack-defense tree model for attacks on Bitcoin. (2) We demonstrate the general usability of existing analysis methods for attack-defense trees in this context. (3) We highlight further research directions necessary to extend attack-defense trees to a full-fledged overarching model for security assessment.