Built by the Mughal Emperor Akbar, the 16th-century Allahabad fort on the northern bank of the Yamuna has massive walls with three gateways flanked by towers. Most of it is occupied by the Indian army and cannot be visited, but a small door in the eastern wall by Sangam leads to one part you can enter: the Patalpuri Temple.
This unique underground temple is crowded with all sorts of idols; pick up some coins from the change dealers outside so you can leave small offerings as you go. (You may be pressured into giving ₹10 to ₹100 at some shrines, but a few coins are perfectly acceptable.)
Outside the temple is the Undying Banyan Tree. Pilgrims used to leap to their deaths from it, believing this would liberate them from the cycle of rebirth. The tree's roots form one of the shrines in the underground temple.