The Forretress (56 / 144) card can be found in the Skyridge set.
Recently sold 2 times and based on those sales is worth around £15.04.
Currently, there is 1 open listing for this card on ebay.
This card sells in the range of £14.51 to £15.57, making it very stable.
This Common card was illustrated by Kouki Saitou.