WordNet sense 1 (able to withstand attack):

impregnable, inviolable, strong, unassailable, unattackable

WordNet sense 2 (financially sound):

dependable, good

WordNet sense 3 (financially secure and safe):

sound

WordNet sense 4 (get by special effort):

procure

WordNet sense 5 (securely or solidly fixed in place; rigid):

tight

WordNet sense 6 (securely in position; not shaky):

steady

WordNet sense 7 (physically secure or dependable):

sure

WordNet sense 8 (in safekeeping):

safe

WordNet sense 9 (securely established):

firm, established

WordNet sense 10 (characterized by certainty or security):

assured

WordNet sense 11 (free from fear or doubt; easy in mind):

untroubled, unafraid

WordNet sense 12 (protected from danger or bad weather):

sheltered

WordNet sense 13 (secured with bastions or fortifications):

fortified

WordNet sense 14 (kept safe or defended from danger or injury or loss):

protected

WordNet sense 15 (fill or close tightly with or as if with a plug):

plug, stop up

WordNet sense 16 (cause to be firmly attached):

fasten, fix

WordNet sense 17 (furnish with battens):

batten, batten down

WordNet sense 18 (make certain of):

assure, ensure, guarantee, insure