Firewall is an essential device in every computer network. It needs skillful professionals to accurately configure its rules for proper functioning. To help prepare these professionals, university level students need more engaging and attractive interactive tools to develop their skills. For this regard, this paper presents the design, implementation and evaluation of "Compu Castel" educational video game that teaches firewall concepts. In addition to evaluating the impact of educational game on short-term knowledge acquisition, both, mid-term (after 2 months) and long-term (after 5 months) knowledge retention is analyzed. The results confirm that educational games affect positively short-term knowledge acquisition compared with traditional text based methods. Moreover, educational games enhance knowledge retention for mid-term and long-term periods.