The refuge is one of the biggest wintering places for bats. Number of bat population within the fortress has increased after people stopped using it. Individuals of the following species regularly winter here:

• pond bat
• greater mouse-eared bat

Particularly valuable species in danger of extinction and included in the Polish Red Data Book of Animals is pond bat. Twierdza Wisloujscie is one of the four biggest refuges of this species in Poland.