If you don't have a unit reaching J-112 (the room Colette is in), she'll die and you won't be able to recruit her later. If you do get there, she'll run off to the next stage and be safe (for a while).
Before turn 20, areas south of B-110 and west of S-108 are inaccessible (the yellow lines are solid walls). The sealed off areas do not count towards occupation ratio, so it is possible to reach 100% occupation within 19 turns.
On turn 20, the entire J-116 square will disappear, while B-111 and below become revealed. Occupation ratio is recalculated based on the accessible squares.
On turn 40, the entire B-116 square will disappear, while R-108 and below are revealed. Occupation ratio is recalculated based on the accessible squares.
It is important to know that depended on the path you capture that will be your starting point at stage 03-02
There are generally no benefits from exploring the additional areas that open up in the later turns, with the exception of a single magic card at R-108. It is highly recommended to not bother with the magic card, and capture either the center or the right (East) castles. This is because for the next stage, those two positions have a significant advantage in rescuing/recruiting Colette in 03-02, a very useful unit. Waiting to acquire the R-108 would result in starting 03-02 from the left (West) castle, which might make rescuing Colette difficult or impossible.
The treasure chest at Z-109 only appears when you are replaying the level. It contains a long sword.
J-98 = Orange Nut
E-100 = Orange Nut
I-101 = Green Nut
K-101 = Healing Water (Small)
K-102 = Magician's Solution
K-108 = Orange Nut
I-110 = Water of Courage (勇壮の水)
I-109 = Bronze Coin
C-106 = Battle Aze
Z-109 = Long Sword (replay only)
Y-110 = Yellow Nut
W-109 = Corushino Steel (コルシノ鋼)
Firame spawns at the three portals on turns 10, 20, 30, 40 (not 50, 60, or 70)