1. Yeah you kinda have to learn to live with those obstructions. Use other exits or blow it up.
2. I cannot control the differences between difficulty levels that precisely. 2 is indeed pretty easy due to minimal spawns. 3 is 'likely' to produce a close-to-minimal spawn, but if you're unlucky, it can generate any spawn - up to maximum spawn. 4 is just more likely to produce maximum spawn, but as above, it can, if you're lucky, generate a small spawn. Only 5th difficulty guarantees maximum spawn. Also, enemies get stat buffs starting with difficulty 2, but these don't make a big difference between two neighbouring difficulties. For completeness' sake, difficulty 1 is guaranteed to have minimum spawns PLUS all enemies have half armor, so it's really a kindergarten difficulty. Spawn sizes in Piratez vary a lot between minimum and maximum (minimum spawn is 25-50% of maximum spawn) so diff 3 can kick your as, albeit only ocasionally.
