Genesis block with the hash block carrying a similar replica of the genesis block.

Hash functions:

Properties:

Blockchain is protected by the properties of hash functions.

Decentralisation of the blockchain

#Blockchains are based on an active peer 2 peer network which interchanges every single block. Everyone has his own copy of the whole blockchain. 

Brute forcing takes a lot of time and resources... changing one thing will result in a different result and you will be able to pick up the change.

Orphan blocks & mem pools. where transactions on the blockchain are verified. High fees is what the miners carry out first.

Hash's and infeasability... having to crack a number of hash's to change a transaction.