An obsolete dreadnought as a Tobruk fortress

Could an obsolete battleship have helped defend Tobruk?

Yes; beached and reinforced as a fixed gun platform, it could impose a major bombardment and siege problem.

Removing the requirement to float changes the survivability problem. An obsolete dreadnought could be deliberately beached in Tobruk harbor rather than preserved as a mobile warship. Empty compartments could be filled with concrete and covered with additional steel or earth, making ordinary sinking damage less relevant. Its heavy guns would then become a persistent coastal-fire and siege obstacle that attackers had to suppress or bypass.