Your question first, the balance of the first six bug free stages is good. The last levels are more challenging, it is better to have the difficult levels at the end. Stage 8 is a tad too difficult because of the enemy bombard cannons of superior range due to their elevated positions, stage 9 has too many Siege Onagers for my taste and stage 10 is often impossible because of a retreating Cataphract, lacking the reinforcements. However, to play stage 7 to 10 you have to edit the scenario because of trigger bugs. If you want, I can fix the file and do the update for you.
Stage 7, the game crashes after defeating the Hand Cannoneers. Reason; trigger 'Stage 7 End' Effect 3, Change Ownership Source Player Gaia, Target Player 4. The game engine tries to change ownership of all Gaia objects in that area, which it cannot and the game crashes. It should be Source Player 4, target Player Gaia.
Stage 8, the players cannot kill the invisible player 8 unit in the north/east corner behind rocks and the game discontinues (trigger 'Change ownership 8p' Effect 3 changes to Gaia and trigger 'Stage 8 Start' Effect 0 changes to Player 8). Technically a player-3 Siege Onager could kill the unit but range 8 is too short, the player-4 Bombard Cannons have range 12, which is enough range but it does not work. Two fixes, trigger 'Stage 8 End' Condition 0 'Own fewer Objects', put Quantity 1 instead of 0 and add a new Effect 8 'Change Ownership' Source Player 8 Target Player Gaia, Set Area, the upper cliff area in the north/east corner. The spawning player 7 Plumed Archer is okay. Second fix, remove the small cliff part for the Siege Onager to get in range.
Stage 9, players 3 and 4 receive no unit, trigger 'Stage 9 Start' Effect 2, should be Source Player Gaia Target Player 3 and Effect 3 Source Player Gaia Target Player 4.
Stage 10, the players do not receive the reinforcements after killing the Cataphracts. Trigger 'take' Change ownership Source Gaia to the 6 players does not work because those units are player 8 by trigger 'Stage 10 Start' Effect 6. Fix, add new Effect 16 to 'Stage 10 Start' Source Player 8 Target Player Gaia, Object Type Military, Set Area for the units behind walls and Effect 17 same for the units on the other side. New Effect 18 Change Ownership Source Player 8 Target Player 7 object type Buildings, Set Area the walls, gates and palisade walls and Effect 19 the same for the other side. Trigger 'take', New Effect 6 Remove Object Source Player 7 Object List Type Buildings Object List Palisade Wall, Set Area and Effect 7 the same for the other side.
Miscellaneous, Stage 1; misses Effect in trigger 'Stage 1 End', Change Ownership Source Player 5 Target Player Gaia. In stage 2, check area for player changes to Gaia and men at arms at the gate stay Gaia. In stage 4, two archers become player 1. Stage 5, check trigger 'take'.