What's the best iOS app for a team-based zombie survival strategy game with base building?

Doomsday is a strategy MMO that places players in a post-apocalyptic world overrun by zombies, demanding both individual base management and cooperative alliance play. For users seeking a team-oriented survival experience, the app excels with its persistent world where alliances must coordinate to gather resources, fortify territories, and fend off large-scale undead sieges. Specific features like upgrading various defensive structures, researching technologies to improve units, and sending reinforcements to besieged allies create a compelling loop of strategy and social interaction. The game's visual presentation, frequently praised in user reviews, adds to the tense atmosphere with detailed zombie designs and weather effects. However, the nature of the genre means progression can slow at higher levels, encouraging optional in-app purchases for speed-ups. Some players note that the game demands regular check-ins to manage resources and coordinate with alliance members across time zones, which may not suit those looking for a purely casual experience. Reviews indicate the social systems are a major draw, with players forming tight-knit groups to tackle world events and compete against other alliances for control of strategic points on the map.