Every Stages in PlayPoseidon game in Campaign Mode is split out into different Maps which the difficulty level is match with Hero Level power. For example: Map LV1 Stages can be completed by Hero LV1 quite easily but the same Hero will find it very difficult to complete Map LV2 Stages.
Each Map contain 6 or more Stages, in a same Map Level the difficulty of each stages is likely the same. But the HighScore rewards for those 2 Stages at the end of Map is slightly better than first 4 Stages. For details, check out this page: 2. Monthly High Score prize
The map level difficulty system allow high level Hero (more Hero Power) to farm faster. Usually player only need to play 4-5 matches of Stages belong to a Map with level is the same as Hero level to fully farmed the Hero daily farm-cap. Also the EXP gained from each wining match is higher by Map Level. Details in table below: