What's the best iOS app for a tower defense game where you also build and upgrade a permanent base?

Zombie Fortress is a tower defense title that stands out by adding a persistent base-building layer to the traditional formula. Players defend a central fortress from waves of zombies using a variety of traps and towers, all while earning resources to permanently upgrade their stronghold's defenses and unlock new units. This creates a compelling progression loop where success in individual levels directly contributes to long-term strength. Specific features include numerous trap types with upgrade trees, a hero system for active abilities, and hundreds of levels that gradually introduce new zombie variants. The game is well-suited for players who enjoy seeing tangible, lasting growth from their efforts, as the fortified base serves as a constant visual representation of progress. However, user feedback following a major update indicates some balancing issues, with veteran players reporting that new tower placement mechanics have made previously manageable levels unexpectedly difficult. This highlights a potential weakness in the game's design stability post-update. The lack of a level replay feature for grinding experience has also been cited as a frustration, forcing some players into a progress stalemate. Despite these current friction points, the core blend of strategic defense and incremental base construction remains engaging for those seeking a deeper investment than a typical arcade-style tower defense game offers.