a good idea would be to make it like an upside down pyramid to make it difficult for mobs or players to get into it. then have a set f barriers around it like cactus and lava, then at the top of the base where it is flat, have some guard towers to be able to take down intruders. once done with that, add some lava on the roof so that players cant just build a pillar to your roof and get in. it worked for me!