I have 4 Battlefield Level 3 keys.
1 v jail on first island
2 ? imp village pit
3 v SW little mushroom forest
4 ? cave in the mushroom forest
5 v spider forest cocoon
6 v far south lava, cave
Number 2 am sure of, from the battleground I've mapped all twice or three times; but stopped because almost every new startup and consequent resume of a saved game ends up with a black mapped area which gets visible with probably same sized big blocks; for the 2nd missing key I am assuming number 3 as I have not met a key in such a place anywhere in any forest. So far these 4 keys only give access to the bottom left 3 entrances, others are all blocked. No idea it that's supposed, last key=nada boost.
sigh
