Defence

Defence recruitment runs on two constraints that don't apply elsewhere: security clearance and on-site working. Both shrink the market before technical requirements are considered, and both are routinely underestimated when a brief is written.

Clearance is the harder one. SC takes time and DV takes considerably longer, and while many roles will accept a clearable candidate, "eligible to obtain" carries residency and nationality requirements that remove a large proportion of the available engineers. Firms that need someone productive in month one and firms that can wait for clearance are effectively recruiting in two different markets, and it's worth being explicit about which one you're in.

The work itself is often the strongest selling point. Real-time systems used operationally, hardware that has to work in conditions that don't forgive, and engineering teams small enough that individuals visibly own things. Engineers who want that are usually already in the sector — which makes the market a closed loop, and a specific approach considerably more effective than an advert.