Prepare to lose all your savegames on the PC if this game crashes during a save!
In this game you follow Ezio as an old man but he is evermore agile in his movements. Other things that remain the same are acrobatics, assasinations, side missions, assassin missions and economy. The new toys include custom made bombs, new weapons and a much better weapons GUI.
Other annoying things about this game are horse cart racing which is hard to control and timed races, some of which are compulsory to progress the story.
And obviously the save system. Which is nothing new, back in AC2 I went from sequence 9 to sequence 3 and sequence 12 to sequence 9. Turning off cloud synchronisation and regularly backing up your savegames manually would be a good practice if you don't want to lose progress.