Clan Castle 1 3 3.
But surely will slow down others.
When the army camp is done building, upgrade it to level 2.
Otherwise, you will face the threat of rushing and end up with a low-level Wall base or some Baby Heroes later! I recommend trying not to upgrade more than 1 Mortar or more than 1 Air Defense at once; upgrading both Air Defense towers or multiple Mortars will leave your base vulnerable to certain attacks.
Altogether, you will need to farm 41,060,000 gold in order to max out your walls at this level range.
For example, complete Archer Queen from 40 to 45 first, then bring King from 40 to 45.