1. I chose Castor as the main hero, and I don't even remember on what level he got the Wizard skill, which allows him to move normally. It was towards the end of the first week (around day 6 or 7).
2. I didn't realize how to defeat 5 Titans without dragons. And, accordingly, how to capture the dragonlings without Hydras. I managed to defeat them around day 8.
3. I wasted time collecting artifacts. By the end, they disappeared for no reason because I couldn't carry them all.