Some more info i received from a top player

The Plux™

Sorry had to change my forums name due to having problems logging in Rabitt

Info from a very respected player
1) when robbed you lose 50% of your mafia item defence so you have to use the right defensive buildings, see mine like Missile Turrets, Surveillance Centres, Gattling Guns to fill that defensive gap. Bodyguard Agencies are useless about 1/6 as effective in real battle as missile turrets.
2) Defensive Buildings actually add to your PvP overall defence by half the anti rob effect. This actually adds to your mafia in battle and makes you defensively stronger! Again the right buildings are like super upgradeable mafia defensive items.
3) Pick one nightclub and focus on upgrading that, focus on building def upgrading it and saving as a secondary goal you'll find slowly but surely you get stronger and stronger defensively and as a by-product earn more income in the biggest possible steps.
That's my Plux Method to Crime City