Two ways.

Firstly the game actually allows players of Homelands to play level 1 without having completed level 0. This was because level 0 was considered a bad introduction but was a good level in terms of high scores.... and it would have not been right to remove the level.

Secondly, some levels are impossible. Not in homelands, but in other level sets. In some cases I have made the levels possible in subsequent releases but people who reached them when they were impossible often asked me if I could "put them past it". Plays like "Wil N Wil" have progressed through level sets 5 and 6 like this (since they were full of levels that were impossible playing teamwork).